Sensing and atrial-synchronized ventricular pacing in an intracardiac pacemaker

ABSTRACT

An intracardiac pacemaker is configured to filter a raw cardiac electrical signal received by the pacemaker to produce a filtered cardiac electrical signal, analyzes the filtered cardiac electrical signal to establish cardiac event sensing criteria that discriminate P-waves from T-waves and R-waves all present in the raw cardiac electrical signal, and sense the P-waves from the filtered cardiac electrical signal when the established cardiac event sensing criteria are met. Sensed P-waves may be used for controlling atrial-synchronized ventricular pacing delivered by the pacemaker.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/068,363, filed on Oct. 24, 2014. The disclosure of the aboveapplication is incorporated herein by reference in its entirety.

The present application is related to co-pending and commonly-assignedU.S. patent application Ser. No. ______ (Atty. Docket No.:C00005900.USU2) which is entitled SENSING AND ATRIAL-SYNCHRONIZEDVENTRICULAR PACING IN AN INTRACARDIAC PACEMAKER; and U.S. patentapplication Ser. No. ______ (Atty. Docket No.: C00005900.USU4), which isentitled SENSING AND ATRIAL-SYNCHRONIZED VENTRICULAR PACING IN ANINTRACARDIAC PACEMAKER, both of which are filed concurrently herewithand all of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The disclosure relates to an implantable medical device system andassociated method for sensing cardiac events by an intracardiacpacemaker configured to deliver atrial-synchronized ventricular pacing.

BACKGROUND

Implantable cardiac pacemakers are often placed in a subcutaneous pocketand coupled to one or more transvenous medical electrical leads carryingpacing and sensing electrodes positioned in the heart. A cardiacpacemaker implanted subcutaneously may be a single chamber pacemakercoupled to one medical lead for positioning electrodes in one heartchamber, atrial or ventricular, or a dual chamber pacemaker coupled totwo leads for positioning electrodes in both an atrial and a ventricularchamber. Multi-chamber pacemakers are also available that may be coupledto three leads, for example, for positioning electrodes for pacing andsensing in one atrial chamber and both the right and left ventricles.

Intracardiac pacemakers have recently been introduced that areimplantable within a ventricular chamber of a patient's heart fordelivering ventricular pacing pulses. Such a pacemaker may sense R-wavesignals attendant to intrinsic ventricular depolarizations and deliverventricular pacing pulses in the absence of sensed R-waves. While singlechamber ventricular sensing and pacing by an intracardiac ventricularpacemaker may adequately address some patient conditions, otherconditions may require atrial and ventricular (dual chamber) sensing forproviding atrial-synchronized ventricular pacing and/or atrial andventricular (dual chamber) pacing in order to maintain a regular heartrhythm.

SUMMARY

In general, the disclosure is directed to an intracardiac pacemakercapable of dual chamber sensing for providing atrial-synchronizedventricular pacing therapy to a patient. A pacemaker operating accordingto the techniques disclosed herein filters a raw cardiac signalincluding P-waves, T-waves and R-waves, analyzes at least the filteredcardiac signal to identify P-waves and the T-waves, and establishescardiac event sensing criteria that discriminate the P-waves from theR-waves and T-waves in the filtered cardiac electrical signal. Invarious examples, P-wave sensing by an intracardiac pacemaker enablesatrial-synchronized ventricular pacing by the pacemaker.

In one example, the disclosure provides a method performed by a medicaldevice including filtering a raw cardiac electrical signal received bythe medical device according to first filtering properties to produce afiltered cardiac electrical signal, the raw cardiac electrical signalcomprising first cardiac events, second cardiac events different thanthe first cardiac events, and third cardiac events different than thefirst cardiac events and the second cardiac events, detecting a firstcrossing of a first threshold by the filtered cardiac electrical signal,identifying one of the second cardiac events after the first crossing,detecting a second crossing of the first threshold by the filteredcardiac electrical signal after the identified one of the second cardiacevents, analyzing the first crossing and the second crossing of thefiltered cardiac electrical signal, establishing cardiac event sensingcriteria that discriminate the first cardiac events from the thirdcardiac events based on the analyzing of the first crossing and thesecond crossing, and sensing the first cardiac events from the filteredcardiac electrical signal when the established cardiac event sensingcriteria are met.

In another example, the disclosure provides an implantable medicaldevice, including a sensing module configured to receive a raw cardiacelectrical signal via electrodes coupled to the sensing module, the rawcardiac electrical signal including first cardiac events, second cardiacevents different than the first cardiac events, and third cardiac eventsdifferent than the first cardiac events and the second cardiac events.The sensing module is further configured to filter the raw cardiacelectrical signal according to first filtering properties to produce afiltered cardiac electrical signal, detect a first crossing of a firstthreshold by the filtered cardiac electrical signal, identify one of thesecond cardiac events after the first crossing, detect a second crossingof the first threshold by the filtered cardiac electrical signal afterthe identified one of the second cardiac events, analyze the firstcrossing and the second crossing of the filtered cardiac electricalsignal, establish cardiac event sensing criteria that discriminate thefirst cardiac events from the third cardiac events based on theanalyzing of the first crossing and the second crossing, and sense thefirst cardiac events from the filtered cardiac electrical signal whenthe established cardiac event sensing criteria are met.

In yet another example, the disclosure provides a non-transitory,computer-readable medium storing a set of instructions which, whenexecuted by an implantable medical device cause the device to filter araw cardiac electrical signal received by the medical device accordingto first filtering properties to produce a filtered cardiac electricalsignal, the raw cardiac electrical signal comprising first cardiacevents, second cardiac events different than the first cardiac events,and third cardiac events different than the first and second cardiacevents. The instructions further cause the device to detect a firstcrossing of a first threshold by the filtered cardiac electrical signal,identify one of the second cardiac events after the first crossing,detect a second crossing of the first threshold by the filtered cardiacelectrical signal after the identified one of the second cardiac events,analyze the first crossing and the second crossing of the filteredcardiac electrical signal, establish cardiac event sensing criteria thatdiscriminate the first cardiac events from the third cardiac eventsbased on the analyzing of the first crossing and the second crossing,and sense the first cardiac events from the filtered cardiac electricalsignal when the established cardiac event sensing criteria are met.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem that may be used to sense cardiac electrical signals and providetherapy to a patient's heart.

FIG. 2A is a conceptual diagram of an intracardiac pacemaker.

FIGS. 2B and 2C are conceptual diagrams of alternative embodiments of anintracardiac pacemaker.

FIG. 3 is a functional block diagram of an example configuration of theintracardiac pacemaker shown in FIG. 2A.

FIG. 4A is a functional block diagram of sensing and pacing controlcircuitry included in the right ventricular (RV) pacemaker shown in FIG.1 according to one example.

FIG. 4B is a depiction of raw unfiltered EGM signals received by theP-wave detector of FIG. 4A.

FIG. 4C is a depiction of filtered, rectified EGM signals produced afterfiltering and rectifying the respective unfiltered EGM signals of FIG.4B.

FIG. 5A is a conceptual diagram of a P-wave portion of a ventricular EGMsignal.

FIG. 5B is a conceptual diagram of an R-wave portion of the ventricularEGM signal of FIG. 5A.

FIG. 5C is a conceptual diagram of a T-wave portion of the ventricularEGM signal of FIG. 5A.

FIG. 6 is a flow chart of a method for controlling ventricular pacingpulse delivery using far-field P-wave sensing by RV pacemaker of FIG. 1.

FIG. 7A is a flow chart of a method for controlling ventricular pacingpulse delivery by the RV pacemaker according to another example.

FIG. 7B is a depiction of a raw, unfiltered EGM signal, a filtered,rectified EGM signal, a differential EGM signal and an integrated EGMsignal.

FIG. 8 is a flow chart of a method for establishing and re-checkingP-wave sensing criteria by the RV pacemaker.

FIG. 9 is a flow chart of a method for establishing P-wave sensingcriteria by the RV pacemaker according to one example.

FIG. 10A is a flow chart of a method for determining a need foradjusting P-wave sensing control parameters used by the P-wave detectorshown in FIG. 4A.

FIG. 10B is a flow chart of a method for determining a need foradjusting the P-wave sensing criteria according to another example.

FIG. 11A is a diagram of a cardiac EGM signal and associated markerchannel signals that may be produced by the RV pacemaker and transmittedto an external device according to one example.

FIG. 11B is a conceptual diagram of a display of the cardiac EGM signaltransmitted from the RV pacemaker to the external device and anassociated marker channel that may be generated by a user display of theexternal device.

FIG. 12 is a flow chart of a method that may be performed by the RVpacemaker of FIG. 1 for automatically adjusting a P-wave detector filteraccording to another example.

DETAILED DESCRIPTION

An implantable medical device (IMD) system is disclosed herein thatincludes a ventricular intracardiac pacemaker configured to be implantedwholly in a ventricular chamber of the patient's heart. In variousexamples, the IMD system may include an atrial intracardiac pacemakerand a ventricular intracardiac pacemaker that do not require transvenousleads but are enabled to provide coordinated atrial and ventricularpacing without wireless or wired communication signals between the twointracardiac pacemakers. The ventricular intracardiac pacemakerestablishes P-wave sensing criteria for reliably distinguishing P-wavesfrom R-wave and T-waves to enable the ventricular pacemaker to deliveratrial-synchronized ventricular pacing.

A dual chamber pacemaker positioned in an implant pocket and coupled totransvenous atrial and ventricular leads may be programmed to deliveronly atrial pacing (AAI(R)), only ventricular pacing (VVI(R)) or both(DDD(R)) according to patient need. The dual chamber pacemaker is ableto control the delivery of pacing pulses in both atrial and ventricularchambers because the pacemaker will receive sensed event signals fromboth atrial and ventricular chambers and control when a pacing pulse isdelivered in both chambers relative to the sensed events using theelectrodes positioned in both chambers. In other words, the dual chamberpacemaker knows when both sensed and paced events have occurred in bothatrial and ventricular sensing and pacing channels since all sensing andpacing control is happening in the one device, i.e., the dual chamberpacemaker.

Intracardiac pacemakers have been introduced that are adapted to beimplanted wholly within a heart chamber. Elimination of transvenous,intracardiac leads has several advantages. For example, complicationsdue to infection associated with a lead extending from a subcutaneouspacemaker pocket transvenously into the heart can be eliminated. Othercomplications such as “twiddler's syndrome”, lead fracture or poorconnection of the lead to the pacemaker are eliminated in the use of anintracardiac pacemaker.

An intracardiac pacemaker may operate in a single chamber pacing andsensing mode, e.g., AAI or VVI, by delivering pacing pulses andinhibiting pacing when an intrinsic event is sensed in the chamber thatthe pacemaker is implanted in. While some patients may require onlysingle chamber pacing and sensing, patients having AV conduction defectsmay require a pacing system capable of delivering ventricular pacingpulses that are synchronized to atrial events, including atrial pacedevents (if an atrial pacemaker is present) and sensed intrinsic atrialevents (i.e., P-waves attendant to the depolarization of the atria).When ventricular pacing pulses, are properly synchronized to atrialevents, the ventricle is paced at a target atrioventricular (AV)interval following an atrial event. Maintaining a target AV interval isimportant in maintaining proper filling of the ventricles for promotingoptimal hemodynamic function.

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem 10 that may be used to sense cardiac electrical signals andprovide therapy to a patient's heart 8. IMD system 10 includes a rightventricular (RV) intracardiac pacemaker 14 and may optionally include aright atrial (RA) intracardiac pacemaker 12. Pacemakers 12 and 14 aretranscatheter intracardiac pacemakers adapted for implantation whollywithin a heart chamber, e.g., wholly within the RV, wholly within theleft ventricle (LV), wholly within the RA or wholly within the leftatrium (LA) of heart 8. In the example of FIG. 1, pacemaker 12 ispositioned along an endocardial wall of the RA, e.g., along the RAlateral wall or RA septum. Pacemaker 14 is positioned along anendocardial wall of the RV, e.g., near the RV apex. The techniquesdisclosed herein, however, are not limited to the pacemaker locationsshown in the example of FIG. 1 and other positions and relativelocations from each other are possible. In some examples, a ventricularintracardiac pacemaker 14 is positioned in the LV for deliveringatrial-synchronized ventricular pacing using the techniques disclosedherein.

Pacemakers 12 and 14 are reduced in size compared to subcutaneouslyimplanted pacemakers and are generally cylindrical in shape to enabletransvenous implantation via a delivery catheter. In other examples,pacemakers 12 and 14 may be positioned at any other location inside oroutside heart 8, including epicardial locations. For example, pacemaker12 may be positioned outside or within the right atrium or left atriumto provide respective right atrial or left atrial pacing. Pacemaker 14may be positioned outside or within the right ventricle or leftventricle to provide respective right ventricular or left ventricularpacing.

Pacemakers 12 and 14 are each capable of producing electricalstimulation pulses, i.e., pacing pulses, delivered to heart 8 via one ormore electrodes on the outer housing of the pacemaker. RA pacemaker 12is configured to sense an intracardiac electrogram (EGM) signal in theRA using the housing based electrodes and deliver RA pacing pulses. RVpacemaker 14 is configured to sense an EGM signal in the RV usinghousing based electrodes and deliver RV pacing pulses.

In some examples, a patient may only require RV pacemaker 14 fordelivering atrial-synchronized ventricular pacing, e.g., in the case ofatrio-ventricular (AV) block. In other examples, depending on individualpatient need, RA pacemaker 12 may be implanted first, and RV pacemaker14 may be implanted at a later time after the patient develops a needfor ventricular pacing, e.g., if the patient develops AV conductiondefects. In other examples, the patient may receive the RV pacemaker 14first and later receive RA pacemaker 12, or the patient may receive bothRA pacemaker 12 and RV pacemaker 14 during the same implant procedure.

The RV pacemaker 14 is configured to control the delivery of ventricularpacing pulses to the RV in a manner that promotes maintaining a targetAV interval between atrial and ventricular events, e.g., between P-waves(intrinsic or pacing-evoked) and ventricular pacing pulses or theresulting pacing-evoked R-waves. A target AV interval may be aprogrammed value selected by a clinician. A target AV interval may beidentified as being hemodynamically optimal for a given patient based onclinical testing or assessments of the patient or based on clinical datafrom a population of patients. Each of the RA pacemaker 12 and RVpacemaker 14 include a control module that controls functions performedby the respective pacemaker. According to the techniques disclosedherein, the control module of the RV pacemaker 14 is configured toautomatically adjust P-wave sensing criteria and dynamically adjust aventricular pacing escape interval based on EGM signal analysisperformed to discriminate P-waves from R-waves and T-waves.

Pacemakers 12 and 14 may each be capable of bidirectional wirelesscommunication with an external device 20. Aspects of external device 20may generally correspond to the external programming/monitoring unitdisclosed in U.S. Pat. No. 5,507,782 (Kieval, et al.), herebyincorporated herein by reference in its entirety. External device 20 isoften referred to as a “programmer” because it is typically used by aphysician, technician, nurse, clinician or other qualified user forprogramming operating parameters in pacemakers 12 and 14. Externaldevice 20 may be located in a clinic, hospital or other medicalfacility. External device 20 may alternatively be embodied as a homemonitor or a handheld device that may be used in a medical facility, inthe patient's home, or another location. Operating parameters, such assensing and therapy delivery control parameters, may be programmed intopacemakers 12 and 14 using external device 20.

External device 20 includes a processor 52 and associated memory 53,user display 54, user interface 56 and telemetry module 58. Processor 52controls external device operations and processes data and signalsreceived from pacemakers 12 and 14. According to techniques disclosedherein, processor 52 receives EGM and marker channel data transmitted totelemetry module 58 from RV pacemaker 14. Processor 52 provides userdisplay 54 with the EGM and marker channel data for display to a user.

The user display 54 produces a display of EGM signal data, which may bedelayed from real time as described below in conjunction with FIGS. 11Aand 11B, and marker channel markers based on data received from RVpacemaker 14. The display may be a part of a graphical user interfacethat facilitates programming of sensing control parameters by a userinteracting with external device 20. External device 20 may displayother data and information relating to pacemaker functions to a user forreviewing pacemaker operation and programmed parameters as well as EGMsignals or other physiological data that is retrieved from pacemakers 12and 14 during an interrogation session. User interface 56 may include amouse, touch screen, keyboard and/or keypad to enable a user to interactwith external device 20 to initiate a telemetry session with pacemakers12 and/or 14 for retrieving data from and/or transmitting data topacemakers 12 and/or 14 for selecting and programming desired sensingand therapy delivery control parameters.

Telemetry module 58 is configured for bidirectional communication withimplantable telemetry modules included in each of pacemakers 12 and 14.External device 20 establishes a wireless radio frequency (RF)communication link 22 with RA pacemaker 12 and wireless RF communicationlink 24 with RV pacemaker 14 using a communication protocol thatappropriately addresses the targeted pacemaker 12 or 14. An example RFtelemetry communication system that may be implemented in system 10 isgenerally disclosed in U.S. Pat. No. 5,683,432 (Goedeke, et al.), herebyincorporated herein by reference in its entirety.

Telemetry module 58 is configured to operate in conjunction withprocessor 52 for sending and receiving data relating to pacemakerfunctions via communication link 22 or 24. Communication links 22 and 24may be established between respective RA pacemaker 12 and RV pacemaker14 and external device 20 using an RF link such as BLUETOOTH®, Wi-Fi,Medical Implant Communication Service (MICS) or other RF bandwidth. Insome examples, external device 20 may include a programming head that isplaced proximate pacemaker 12 or 14 to establish and maintain acommunication link, and in other examples external device 20 andpacemakers 12 and 14 may be configured to communicate using a distancetelemetry algorithm and circuitry that does not require the use of aprogramming head and does not require user intervention to maintain acommunication link.

It is contemplated that external device 20 may be in wired or wirelessconnection to a communications network via telemetry module 58 fortransferring data to a remote database or computer to allow remotemanagement of the patient 12. Remote patient management systems may beconfigured to utilize the presently disclosed techniques to enable aclinician to review EGM and marker channel data and authorizeprogramming of sensing control parameters after viewing a visualrepresentation of EGM and marker channel data. Reference is made tocommonly-assigned U.S. Pat. No. 6,599,250 (Webb et al.), U.S. Pat. No.6,442,433 (Linberg, et al.), U.S. Pat. No. 6,418,346 (Nelson, et al.),and U.S. Pat. No. 6,480,745 (Nelson, et al.) for general descriptionsand examples of remote patient management systems that enable remotepatient monitoring and device programming. Each of these patents isincorporated herein by reference in their entirety.

For example, neither RA pacemaker 12 nor RV pacemaker 14 may beconfigured to initiate an RF communication session with the otherdevice. Both pacemakers 12 and 14 may be configured to periodically“listen” for a valid “wake up” telemetry signal from external device 20and power up its own telemetry module to establish a communication link22 or 24 in response to a valid telemetry signal (or go back to “sleep”if no valid telemetry signal is received). However, pacemaker 12 andpacemaker 14 may or may not be configured to communicate directly witheach other. In some cases, pacemakers 12 and 14 may be configured tocommunicate with each other, but, in order to conserve battery life ofthe intracardiac pacemakers 12 and 14, communication may be minimized.As such, communication does not occur on a beat-by-beat basis betweenthe RA pacemaker 12 and RV pacemaker 14 for communicating when the otherpacemaker is sensing cardiac events or when it is delivering pacingpulses. RV pacemaker 14, however, is configured to sense atrial eventsand automatically adjust P-wave sensing criteria for reliablydiscriminating P-waves from R-waves and T-waves of an intracardiacventricular EGM signal, without requiring communication signals from RApacemaker 12.

FIG. 2A is a conceptual diagram of an intracardiac pacemaker 100 thatmay correspond to RA pacemaker 12 or RV pacemaker 14 shown in FIG. 1.Pacemaker 100 includes electrodes 162 and 164 spaced apart along thehousing 150 of pacemaker 100 for sensing cardiac EGM signals anddelivering pacing pulses. Electrode 164 is shown as a tip electrodeextending from a distal end 102 of pacemaker 100, and electrode 162 isshown as a ring electrode along a mid-portion of housing 150, forexample adjacent proximal end 104. Distal end 102 is referred to as“distal” in that it is expected to be the leading end as it advancedthrough a delivery tool, such as a catheter, and placed against a targetpacing site.

Electrodes 162 and 164 form an anode and cathode pair for bipolarcardiac pacing and sensing. Electrodes 162 and 164 may be positioned onor as near as possible to respective proximal and distal ends 104 and102 to increase the inter-electrode spacing between electrodes 162 and164. Relatively greater inter-electrode spacing will increase thelikelihood of sensing far-field (FF) signals occurring in a differentheart chamber than the chamber in which pacemaker 100 is implanted. Forexample, an increased inter-electrode spacing between electrodes 162 and164 when pacemaker 100 is used as an RV pacemaker may improve reliablesensing of FF atrial events by pacemaker 100 for use in controlling thetiming of ventricular pacing pulses.

In alternative embodiments, pacemaker 100 may include two or more ringelectrodes, two tip electrodes, and/or other types of electrodes exposedalong pacemaker housing 150 for delivering electrical stimulation toheart 8 and sensing EGM signals. Electrodes 162 and 164 may be, withoutlimitation, titanium, platinum, iridium or alloys thereof and mayinclude a low polarizing coating, such as titanium nitride, iridiumoxide, ruthenium oxide, platinum black among others. Electrodes 162 and164 may be positioned at locations along pacemaker 100 other than thelocations shown.

Housing 150 is formed from a biocompatible material, such as a stainlesssteel or titanium alloy. In some examples, the housing 150 may includean insulating coating. Examples of insulating coatings include parylene,urethane, PEEK, or polyimide among others. The entirety of the housing150 may be insulated, but only electrodes 162 and 164 uninsulated. Inother examples, the entirety of the housing 150 may function as anelectrode instead of providing a localized electrode such as electrode162. Alternatively, electrode 162 may be electrically isolated from theother portions of the housing 150. Electrode 162 formed along anelectrically conductive portion of housing 150 serves as a return anodeduring pacing and sensing.

The housing 150 includes a control electronics subassembly 152, whichhouses the electronics for sensing cardiac signals, producing pacingpulses and controlling therapy delivery and other functions of pacemaker100. Tip electrode 164 may be coupled via a feedthrough to circuitrywithin control electronics subassembly 152, e.g., a pacing pulsegenerator and sensing module, to serve as the pacing cathode electrode.

Housing 150 further includes a battery subassembly 160, which providespower to the control electronics subassembly 152. Battery subassembly160 may include features of the batteries disclosed in commonly-assignedU.S. Pat. No. 8,433,409 (Johnson, et al.) and U.S. Pat. No. 8,541,131(Lund, et al.), both of which are hereby incorporated by referenceherein in their entirety.

Pacemaker 100 may include a set of fixation tines 166 to securepacemaker 100 to patient tissue, e.g., by actively engaging with theventricular endocardium and/or interacting with the ventriculartrabeculae. Fixation tines 166 are configured to anchor pacemaker 100 toposition electrode 164 in operative proximity to a targeted tissue fordelivering therapeutic electrical stimulation pulses. Numerous types ofactive and/or passive fixation members may be employed for anchoring orstabilizing pacemaker 100 in an implant position. Pacemaker 100 mayinclude a set of fixation tines as disclosed in commonly-assigned,pre-grant publication U.S. 2012/0172892 (Grubac, et al.), herebyincorporated herein by reference in its entirety.

Pacemaker 100 may further include a delivery tool interface 158.Delivery tool interface 158 may be located at the proximal end 104 ofpacemaker 100 and is configured to connect to a delivery device, such asa catheter, used to position pacemaker 100 at an implant location duringan implantation procedure, for example within a heart chamber.

A reduced size of pacemaker 100 enables implantation wholly within aheart chamber. In FIG. 1, RA pacemaker 12 and RV pacemaker 14 may havedifferent dimensions. For example, RA pacemaker 12 may be smaller involume than pacemaker 14, e.g., by reducing battery size, to accommodateimplantation in the smaller heart chamber. As such, it is recognizedthat pacemaker 100 may be adapted in size, shape, electrode location orother physical characteristics according to the heart chamber orlocation in which it will be implanted.

FIG. 2B is a conceptual diagram of an alternative embodiment of anintracardiac pacemaker 110. Pacemaker 110 includes a housing 150,control electronics subassembly 152, battery subassembly 160, fixationmember 166 and electrode 164 along a distal end 102, and may include adelivery tool interface 158 along the proximal end 104 as describedabove in conjunction with FIG. 2A. Pacemaker 110 is shown to include anelectrode 162′ extending away from housing 150 along an extender 165. Assuch, instead of carrying a pair of electrodes along the housing 150,which limits the maximum possible inter-electrode spacing, an extender165 may be coupled to the housing 150 for positioning the anodeelectrode 162′ at an increased inter-electrode distance from distal tipelectrode 164. The increased distance may position a sensing electrodein or near the atrium for improved P-wave sensing. The techniquesdisclosed herein may be implemented in a pacemaker with a proximalsensing extension as generally disclosed in U.S. Pat. Application No.62/025,690 (Atty. Docket No. C00005334.USP1), filed provisionally onJul. 17, 2014, incorporated herein by reference in its entirety.

FIG. 2C is a conceptual diagram of an alternative embodiment ofintracardiac pacemaker 120 having extender 165 coupled to the distal end102 of pacemaker housing 150 to extend distal electrode 164′ away fromelectrode 162 positioned along housing 150 near or at proximal end 104.Extender 165 is an insulated electrical conductor that may electricallycouple electrode 164′ to pacemaker circuitry via an electricalfeedthrough crossing housing 150. Pacemaker 120 having an insulated,electrically conductive extender 165 for increasing the inter-electrodespacing may correspond generally to the implantable device and flexibleconductor disclosed in commonly-assigned, pre-grant U.S. Publication No.2013/0035748 (Bonner, et al.), hereby incorporated herein by referencein its entirety.

FIG. 3 is a functional block diagram of an example configuration ofpacemaker 100 shown in FIG. 2A. Pacemaker 100 includes a pulse generator202, a sensing module 204, a control module 206, memory 210, telemetrymodule 208 and a power source 214. As used herein, the term “module”refers to an application specific integrated circuit (ASIC), anelectronic circuit, a processor (shared, dedicated, or group) and memorythat execute one or more software or firmware programs, a combinationallogic circuit, or other suitable components that provide the describedfunctionality. Each of RA pacemaker 12 and RV pacemaker 14 will includesimilar modules as represented by the pacemaker 100 shown in FIG. 3;however it is understood that the modules are configured differently asneeded to perform the functionality of the separate RA and RV pacemakers12 and 14.

When pacemaker 100 is configured to operate as RV pacemaker 14, controlmodule 206 is configured to set various ventricular pacing escapeintervals used to control delivery of ventricular pacing pulses asdisclosed herein. When pacemaker 100 is embodied as RA pacemaker 12,control module 206 is configured to set atrial pacing escape intervalsto control delivery of RA pacing pulses. Adaptations of the hardware,firmware or software of the various modules of pacemaker 100 necessaryto meet the described functionality of the intracardiac pacemakerspositioned in different heart chambers as disclosed herein is understoodto be included in the various modules of pacemaker 100 according to theintended implant location.

The functions attributed to pacemaker 100 herein may be embodied as oneor more processors, controllers, hardware, firmware, software, or anycombination thereof. Depiction of different features as specificcircuitry or modules is intended to highlight different functionalaspects and does not necessarily imply that such functions must berealized by separate hardware or software components or by anyparticular architecture. Rather, functionality associated with one ormore modules, processors, or circuits may be performed by separatehardware or software components, or integrated within common hardware orsoftware components. For example, pacing control operations performed bypacemaker 100 may be implemented in control module 206 executinginstructions stored in associated memory 210 and relying on input fromsensing module 204.

The functional operation of pacemaker 100 as disclosed herein should notbe construed as reflective of a specific form of software or hardwarenecessary to practice the methods described. It is believed that theparticular form of software, hardware and/or firmware will be determinedprimarily by the particular system architecture employed in thepacemaker 100 and by the particular sensing and therapy deliverymethodologies employed by the pacemaker 100. Providing software,hardware, and/or firmware to accomplish the described functionality inthe context of any modern pacemaker system, given the disclosure herein,is within the abilities of one of skill in the art.

Pulse generator 202 generates electrical stimulation pulses that aredelivered to heart tissue via electrodes 162 and 164. Electrodes 162 and164 may be housing-based electrodes as shown in FIG. 2A, but one or bothelectrodes 162 and 164 may alternatively be carried by an insulated,electrical conductor extending away from the pacemaker housing asdescribed in conjunction with FIGS. 2B and 2C.

Pulse generator 202 may include one or more capacitors and a chargingcircuit to charge the capacitor(s) to a programmed pacing pulse voltage.At appropriate times, as controlled by a pace timing and control moduleincluded in control module 206, the capacitor is coupled to pacingelectrodes 162 and 164 to discharge the capacitor voltage and therebydeliver the pacing pulse. Pacing circuitry generally disclosed in theabove-incorporated U.S. Pat. No. 5,507,782 (Kieval, et al.) and incommonly assigned U.S. Pat. No. 8,532,785 (Crutchfield, et al.), both ofwhich patents are incorporated herein by reference in their entirety,may be implemented in pacemaker 100 for charging a pacing capacitor to apredetermined pacing pulse amplitude under the control of control module206 and delivering a pacing pulse.

Control module 206 controls pulse generator 202 to deliver a pacingpulse in response to expiration of a pacing escape interval according toprogrammed therapy control parameters stored in memory 210. The pacetiming and control module included in control module 206 includes anescape interval timer that is set to various pacing escape intervalsused for controlling the timing of pacing pulses relative to a paced orsensed event. Upon expiration of a pacing escape interval, a pacingpulse is delivered. If a cardiac event is sensed during the pacingescape interval by sensing module 204, the scheduled pacing pulse may beinhibited, and the pacing escape interval may be reset to a new timeinterval. Control of pacing escape intervals by control module 206 isdescribed below in conjunction with the various flow charts presentedherein.

Sensing module 204 includes cardiac event detectors 222 and 224 forreceiving cardiac EGM signals developed across electrodes 162 and 164. Acardiac event may be sensed by sensing module 204 when the EGM signalcrosses a sensing threshold of a cardiac event detector 222 or 224. Thesensing threshold may be an auto-adjusting sensing threshold that may beinitially set based on the amplitude of a sensed event and decays at apredetermined decay rate thereafter. In response to a sensing thresholdcrossing, sensing module 204 passes a sensed event signal to controlmodule 206.

Sensing module 204 may include a near-field (NF) event detector 222 anda far-field (FF) event detector 224. NF cardiac events are events thatoccur in the heart chamber where the electrodes 162 and 164 are located.FF cardiac events are events that occur in a different heart chamberthan the heart chamber where electrodes 162 and 164 are located.

The NF cardiac event detector 222 of RV pacemaker 12 may be programmedwith a sensing threshold appropriate for sensing R-waves attendant tothe depolarization of the ventricles. NF cardiac event detector 222 ofRV pacemaker 100 produces a NF sensed event signal, also referred toherein as an “R-sense signal,” provided to control module 206 inresponse to detecting an R-wave sensing threshold crossing

The terms “sensed cardiac events” or “sensed events” as used hereinrefer to events sensed by sensing module 204 in response to the EGMsignal crossing a sensing threshold, which may be an amplitudethreshold, a frequency threshold, a slew rate threshold, or anycombination thereof. Sensed cardiac events may include intrinsic eventsand evoked events caused by a delivered pacing pulse. Intrinsic eventsare events arising in the heart in the absence of a pacing pulse.Intrinsic events include intrinsic P-waves, such as sinus P-wavesoriginating from the sinoatrial node of the heart, and intrinsicR-waves, such as sinus R-waves conducted through the heart's normalconduction pathway to the ventricles from the atria via theatrioventricular node. Intrinsic events can also include non-sinusintrinsic events, such as premature atrial contractions (PACs) orpremature ventricular contractions (PVCs) that arise intrinsically fromthe heart but are ectopic in origin.

FF event detector 224 may be configured to sense FF atrial events whenpacemaker 100 is embodied as RV pacemaker 14. A FF atrial event sensingthreshold may be used by FF event detector 224 for sensing FF atrialevents. The FF atrial event sensing threshold is different than thesensing threshold used by NF event detector 222 but is applied to thesame EGM signal developed across electrodes 162 and 164 to enablesensing module 204 to distinctly sense FF atrial events and NF R-waves.FF event detector 224 produces a FF sensed event signal, also referredto herein as a “P-sense signal,” passed to control module 206 inresponse to sensing a FF atrial event. FF atrial events sensed by FFevent detector 224 may include atrial pacing pulses delivered by RApacemaker 12 and/or P-waves, intrinsic or evoked. The FF event detector224 may or may not be configured to discriminate between sensed FFatrial events that are pacing pulses and sensed FF atrial events thatare P-waves. The atrial events sensed by RV pacemaker 14 are referred toas “far-field” events because they are events occurring in a heartchamber different than the RV, where pacemaker 14 is implanted. It isrecognized that when a proximal sensing extension is used as show inFIG. 2B, at least one electrode may be positioned proximate the atriumso that the signal itself may approach near-field atrial signal sensing.

FF P-waves are relatively small amplitude signals compared to NF R-wavesand may be similar in amplitude to NF T-waves, associated with therepolarization of the ventricular myocardium. P-waves may be challengingto distinguish from T-waves and baseline noise on the ventricular EGMsignal. The inter-electrode spacing of sensing electrodes 162 and 164may be increased to enhance sensing of small amplitude FF P-waves by FFevent detector 224. As described in conjunction with the flow chartsdisclosed herein, RV pacemaker 14 is configured to establish P-wavesensing criteria to reliably sense FF P-waves from the ventricular EGMsignal received by sensing module 204 via electrodes 162 and 164.

FF P-waves may be distinguishable from NF R-waves and T-waves based onamplitude, timing, frequency content, slope, the shape of the overallP-wave morphology or specific features of the P-wave morphology. AnR-wave sensing threshold used by NF event detector 222 in RV pacemaker14 may be set greater than an expected FF P-wave amplitude so thatR-waves are sensed when the EGM signal developed across electrodes 162and 164 crosses the R-wave sensing threshold. FF P-waves may be sensedby FF event detector 224 of the RV pacemaker 14 using a differentsensing threshold that is lower than the NF R-wave sensing threshold.

When available, P-sense signals produced by FF event detector 224 in RVpacemaker 14 may be used by control module 206 of RV pacemaker 14 todeliver atrial-synchronized ventricular pacing. FF atrial events,however, may be absent or undersensed by RV pacemaker 14, e.g., due tochanges in position of the electrodes 162 and 164 or other conditionsthat alter the P-wave morphology. In some cases, the T-wave and theP-wave may become indistinguishable. Using the techniques disclosedherein, RV pacemaker 14 is configured to adjust P-wave sensing criteriaused by FF event detector 224 to improve the reliability of P-wavesensing over time and control ventricular pacing in a manner thatmaintains a target AV interval when reliable P-sense signals areproduced by FF event detector 224 and switch to a VVI or VVIR pacingmode when reliable P-sense signals are not available.

Memory 210 may include computer-readable instructions that, whenexecuted by control module 206 and/or sensing module 204, cause controlmodule 206 and/or sensing module 204 to perform various functionsattributed throughout this disclosure to pacemaker 100. Thecomputer-readable instructions may be encoded within memory 210. Memory210 may include any non-transitory, computer-readable storage mediaincluding any volatile, non-volatile, magnetic, optical, or electricalmedia, such as a random access memory (RAM), read-only memory (ROM),non-volatile RAM (NVRAM), electrically-erasable programmable ROM(EEPROM), flash memory, or other digital media with the sole exceptionbeing a transitory propagating signal. Memory 210 stores timingintervals, counters, or other data used by control module 206 to controlthe delivery of pacing pulses by pulse generator 202, e.g., by setting apacing escape interval timer included in control module 206, accordingto the techniques disclosed herein.

Pacemaker 100 may further include one or more physiological sensors 212used for monitoring the patient. Sensors 212 may include a pressuresensor, an acoustical sensor, an oxygen sensor, or any other sensor usedto monitor a patient. In some examples, physiological sensors 212include at least one physiological sensor producing a signal indicativeof the metabolic demand of the patient. The signal indicative of thepatient's metabolic demand is used by control module 206 for determininga sensor indicated pacing rate used to control the heart rate to meetthe patient's metabolic demand.

For example, sensors 212 may include an accelerometer for producing apatient activity signal passed to control module 206. An accelerometerincluded in sensors 212 may be embodied as a piezoelectric crystal forproducing a signal correlated to patient body motion. The use of anaccelerometer in an intracardiac device for obtaining a patient activitysignal is generally disclosed in U.S. patent application Ser. No.14/174,514 filed on Feb. 6, 2014 (Nikolski, et al.), incorporated hereinby reference in its entirety. The use of a patient activity signal forproviding rate-responsive pacing is generally disclosed in U.S. Pat. No.7,031,772 (Condie, et al.), incorporated herein by reference in itsentirety.

In other examples, sensors 212 may include a posture sensor fordetecting changes in patient body posture. A multi-dimensionalaccelerometer for detecting patient posture changes is generallydisclosed in in U.S. Pat. No. 5,593,431 (Sheldon), incorporated hereinby reference in its entirety. Posture changes may be detected for use intriggering an evaluation of P-wave sensing criteria used by the FF eventdetector 224.

Power source 214 provides power to each of the other modules andcomponents of pacemaker 100 as required. Control module 206 may executepower control operations to control when various components or modulesare powered to perform various pacemaker functions. Power source 214 mayinclude one or more energy storage devices, such as one or morerechargeable or non-rechargeable batteries. The connections betweenpower source 214 and other pacemaker modules and components are notshown in FIG. 3 for the sake of clarity.

Telemetry module 208 includes a transceiver and associated antenna fortransferring and receiving data via a radio frequency (RF) communicationlink. Telemetry module 208 may be capable of bi-directionalcommunication with external device 20 (FIG. 1) as described above.

FIG. 4A is a functional block diagram 250 of sensing and pacing controlcircuitry included in RV pacemaker 14 according to one example. RVpacemaker 14 includes an R-wave detector 252, which may correspond to NFevent detector 222 in FIG. 3, and P-wave detector 262, which maycorrespond to FF event detector 224 in FIG. 3. Both R-wave detector 252and P-wave detector 262 receive the ventricular EGM signal developedacross electrodes 162 and 164, which may be passed through a pre-filter251, e.g., 2.5 to 5 Hz high pass filter or a wideband filter having apassband of 2.5 Hz to 100 Hz to remove DC offset and high frequencynoise. The signal passed by pre-filter 251 is referred to herein as a“raw cardiac electrical signal” or “raw unfiltered EGM signal” 265because the signal 265 is minimally filtered by a wideband filter and isnot yet filtered by a filter included in P-wave detector 262 or R-wavedetector 252. In other words, optimal filtering for sensing cardiacelectrical signals has not been applied to the raw, unfiltered EGMsignal 265. Pre-filter 251 may pass a differential or single-endedsignal to R-wave detector 252 and P-wave detector 262.

R-wave detector 252 includes a bandpass filter 254 for filtering the rawunfiltered EGM signal 265 within a bandwidth including expected R-wavefrequencies. R-wave detector filter 254 passes a filtered signal to ananalyzer 256 that may include a sense amplifier or other event detectorthat compares the filtered EGM signal to R-wave sensing criteria.Expected R-wave frequencies may be in the range of approximately 30 Hzto 50 Hz. Filter 254 may have a center frequency between 30 Hz and 50 Hzand a bandwidth as low as 20 Hz and as high as 70 Hz in one example. AnR-wave may be sensed by analyzer 256 when the amplitude of the filteredEGM signal crosses an R-wave sensing threshold, which may beauto-adjusting sensing threshold. R-wave detector 252 passes an R-sensesignal 258 to pace timing and control module 270, which may be includedin control module 206 of FIG. 3. The R-sense signal 258 may also bepassed to P-wave detector 262 for use by analyzer 268 for sensing anddiscriminating P-waves (far-field events) from R-waves and T-waves (bothnear field events).

Pace timing and control module 270 may include a pacing escape intervaltimer that is set to a ventricular pacing escape interval in response toreceiving sense event signals from R-wave detector 252 and P-wavedetector 262. In response to receiving R-sense signal 258 from R-wavedetector 252, the pace timing and control module 270 inhibits ascheduled pacing pulse and resets the pacing escape interval timer to aVV pacing escape interval. The VV pacing escape interval is set tocontrol the ventricular rate. If the VV pacing escape interval expiresbefore pace timing and control module 270 receives an R-sense signal 258or P-sense signal 272, a ventricular pacing pulse is delivered by pulsegenerator 202 (FIG. 3). Pacing pulse delivery will start another VVpacing escape interval. The VV pacing escape interval may be setaccording to a base pacing rate to provide bradycardia pacing or may beshortened from the base pacing rate interval to a sensor-indicated rateresponse interval to provide a faster pacing rate to meet the patient'smetabolic demand.

It is recognized that R-wave detector 252 may set appropriate blankingand/or refractory intervals to avoid oversensing. For example, after anR-sense signal 258 is produced, a short ventricular blanking interval,e.g., up to 150 ms, may be applied to analyzer 256 such that the sameR-wave is not sensed more than once. A ventricular refractory interval,e.g., up to 500 ms, may be applied after the blanking interval toinhibit T-wave oversensing leading to false R-sense signals. In variousexamples, with no limitation intended, a ventricular refractory intervalmay be between 300 ms and 400 ms.

P-wave detector 262 includes a bandpass filter 264 for filtering theraw, unfiltered EGM signal 265 within a bandwidth including expectedP-wave frequencies and may include expected T-wave frequencies andR-wave frequencies. P-wave detector filter 264 may be an adjustablebandpass filter that is automatically adjusted by sensing module 204(under the control of control module 206 in some examples) to promoteseparation of P-wave signals from T-wave and R-wave signals. Separationof P-wave signals from T- and R-wave signals may be based on peak signalamplitude, slew rate or other morphology features, and/or time ofoccurrence in the filtered EGM signal 266.

Generally, T-waves may have a lower frequency, e.g., 10 Hz or lower,than the frequencies of P-waves and R-waves, e.g., 15 Hz or higher and30 Hz or higher respectively. In one example, filter 264 is nominallyadjusted to be a 20 to 70 Hz bandpass filter, but both the high and lowends of the bandpass can be adjusted. However, a “typical” signalfrequency may change with changes in electrode position, electrodespacing or other factors. By adjusting a center frequency and bandwidthfor filter 264, the P-wave signal may be enhanced and the T-wave signalmay be attenuated such that P-wave and T-wave discrimination isreliable. In other words, if the filter bandpass largely eliminates theT-wave from filtered signal 266, a P-wave sensing threshold may be usedto detect P-waves when the amplitude of T-waves is distinctly smallerdue to optimal filtering of T-waves.

In other examples, the center frequency and/or bandpass width of filter264 may be adjusted to intentionally increase the T-wave amplitude infiltered EGM signal 266 so that the P-wave amplitude is distinctly lowerthan the T-wave amplitude. Analyzer 268 receives the filtered EGM signal266 and includes a rectifier for rectifying the filtered EGM signal 266and a sense amplifier or other cardiac event detector that receives therectified, filtered signal for comparison to a P-wave sensing thresholdused for sensing P-waves from the filtered EGM signal 266. The cardiacevent detector included in analyzer 268 may further apply a T-wavesensing threshold that is higher than the P-wave sensing threshold toenable sensing of P-waves when the filtered EGM signal 266 exceeds theP-wave sensing threshold but not the T-wave sensing threshold. A P-sensesignal 272 is produced in response to sensing a P-wave.

It is recognized that P-wave detector 262 may set relevant blankingand/or refractory periods used by analyzer 268 to avoid oversensing. Forexample, after a P-sense signal 272 is produced, an atrial refractoryperiod, e.g., up to 500 ms, may be applied, during which P-sense signals272 are not used by pace timing and control module 270 for setting theescape interval timer. Additionally, P-wave detector 262 may set anatrial blanking period in response to R-sense signal 258 during which noP-sense signals 272 are produced, followed by a post-ventricular atrialrefractory period during which a P-sense signal 272 is ignored by pacetiming and control module 270.

The filtered EGM signal 266 and/or the raw, unfiltered EGM signal 265received from electrodes 162 and 164 may be passed to analyzer 268 forcomparison to P-wave sensing criteria. Analyzer 268 may compare thefiltered EGM signal 266 to amplitude criteria, frequency criteria,timing criteria relative to an R-sense signal 258 received from R-wavedetector 252, morphology criteria, or any combination thereof forsensing a P-wave. Analyzer 268 may include a digital converter forobtaining multi-bit digital EGM signal samples used for determining andcomparing EGM signal features to P-wave sensing criteria.

In some examples, analyzer 268 receives the raw, unfiltered EGM signal265 from electrodes 162 and 164 to perform comparisons between suspectedT-waves and suspected P-waves present in the filtered EGM signal 266 toP-waves and T-waves that can be identified in the unfiltered EGM signal265. Comparisons of the raw, unfiltered EGM signal P-waves and T-wavesmay be used to establish P-wave sensing criteria that distinguishP-waves from T-waves as described in greater detail below. Analyzer 268may automatically adjust the P-wave sensing criteria used by analyzer268 as needed to discriminate P-waves from T-waves based on analysis ofthe raw unfiltered EGM signal 265 and/or filtered EGM signal 266.

In some examples, filter 264 may provide filtered EGM signal 266 andanother alternate filtered EGM signal 267 for comparison to the filteredEGM signal 266 for identifying P-waves and T-waves. For example,alternate filtered EGM signal 267 may have the same or a differentcenter frequency and broader bandwidth than filtered EGM signal 266. Asdescribed below in conjunction with FIG. 8, the filtered EGM signal 266may be analyzed to confirm that P-waves are being discriminated fromT-waves. If P-wave sensing cannot be confirmed based on analyzingfiltered EGM signal 266 alone, the unfiltered EGM signal 265 or analternate filtered EGM signal 267 may be analyzed to identify P-wavesand/or T-waves and compare the occurrence of P-waves and/or T-waves inthe unfiltered or alternate filtered EGM signals 265 and 267,respectively, to the filtered EGM signal 266 to improve P-wave sensingcriteria and confirm P-wave discrimination from T-waves by P-wavedetector 262.

Comparisons between P-waves and T-waves identified on the unfiltered EGMsignal 265 by analyzer 268 may be made to improve discrimination andseparation of P-waves and T-waves in the filtered EGM signal 266. Forexample, sensing module 204 may adjust filter 264 for increasingamplitude separation of P-waves and T-waves and/or adjust a T-wavesensing window to separate P-waves and T-waves based on time.

Comparisons between the P-waves and T-waves within and between theunfiltered EGM signal 265 and filtered EGM signal 266 may be used byanalyzer 268 to automatically adjust the center frequency and/orbandwidth of filter 264 to increase amplitude separation of P-waves andT-waves by reducing the T-wave signal strength and/or increase theP-wave signal strength in the filtered EGM signal 266. The bandpass offilter 264 may be narrowed for example, to reduce the signal strength ofthe T-wave in the filtered EGM signal 266. If the P-wave frequency isapproximately 20 Hz and the T-wave frequency is approximately 10 Hz, forexample, the center frequency of filter 264 may be set at 20 Hz. If theP-wave is relatively narrow, e.g., due to the electrodes being closer tothe atria, a higher center frequency, e.g., 30 Hz may be used. If theP-wave is relatively wide, e.g., due to the electrodes being furtherfrom the atria, a lower center frequency, for example less than 20 Hzmay be used. The bandwidth may be set to attenuate lower frequencies,e.g., 10 Hz and lower, which are more typical of T-waves.

In other examples, filter 264 is adjusted to increase the T-waveamplitude in the filtered EGM signal 266 to provide greater amplitudeseparation between P-waves and T-waves. T-wave amplitude may beintentionally increased by adjusting the center frequency of filter 264to a lower center frequency, e.g., 10 Hz, or increasing its bandwidth inthe lower frequency range, e.g., frequencies less than 15 Hz. If T-wavesand P-waves have similar amplitudes on the unfiltered EGM signal 265 butdifferent signal widths, the P-wave detector filter 264 may be adjustedto increase the T-wave amplitude to allow amplitude thresholds to beused to discriminate between P- and T-waves in the filtered EGM signal266.

Analyzer 268 may set a T-wave sensing window in response to R-sensesignal 258 to encompass a time interval that a T-wave is likely to occur(and a P-wave is less likely to occur) for discriminating T-waves fromP-waves. The start time and/or duration of a T-wave sensing window setin response to the R-sense signal 258 may be adjusted, automatically byanalyzer 268, to separate P-waves from T-waves. For example, analyzer268 may shorten the duration of the T-wave sensing window in response toan increase in the sensed or paced ventricular rate.

A P-sense signal 272 is passed to pace timing and control module 270 inresponse to P-wave sensing criteria being met. Pace timing and controlmodule 270 starts the pacing escape interval timer in response to theP-sense signal 272 by setting the pacing escape interval timer to an AVinterval. As described below, an initial P-sense signal 272 may bepassed to pace timing and control module 270 in response to theamplitude of the filtered EGM signal 266 crossing a P-wave sensingthreshold. The P-wave sensing threshold is set lower in amplitude thanan R-wave sensing threshold. As such, a signal crossing the P-wavesensing threshold may be a P-wave, an R-wave or a T-wave. Analyzer 268analyzes the signal after a crossing of the P-wave sensing thresholdoccurs to confirm that the signal is a true P-wave.

Confirmation of a sensed P-wave may be delayed following the crossing ofthe P-wave sensing threshold due to additional time needed to verifythat the sensed signal is not an R-wave or a T-wave. For example,subsequent to the crossing of the P-wave sensing threshold, analyzer 268may wait for a T-wave sensing threshold crossing and/or an R-wavesensing threshold crossing to verify that the sensed P-wave thresholdcrossing is a true P-wave. During the AV interval started upon theP-sense signal 272 produced when the filtered EGM signal 266 crossed theP-wave sensing threshold, analyzer 268 may determine one or more EGMsignal features such as a peak amplitude, frequency content, slew rate,morphology, timing relative to an R-sense signal 258 and/or performother signal analysis to confirm that the crossing of the P-wave sensingthreshold is a true P-wave.

In some examples, analyzer 265 includes a differentiator and/or anintegrator for producing a differential EGM signal from the rawunfiltered signal 265 and/or from the filtered EGM signal 266 for use inconfirming a sensed P-wave. For example, a slew rate and/or amplitude ofthe differential signal may reliably discriminate a sensed P-wave from aT-wave because the T-wave may be strongly attenuated in the differentialsignal making the slew rate of the differential signal a strongdiscriminator in some instances. In another example, the integratedT-wave signal may be a larger wider signal than the integrated P-wavesignal facilitating identification and clear discrimination of a T-wavefrom a P-wave. Examples of a raw, unfiltered signal 265, a filteredsignal 266, a differential signal and an integrated signal produced byanalyzer 268 are shown in FIG. 7B and described below.

If the signal sensed by P-wave detector 262 is not confirmed as a trueP-wave, a Cancel P-sense signal 274 may be passed to pace timing andcontrol module 270. Pace timing and control module 270 may reset, adjustor cancel the AV pacing escape interval in response to the CancelP-sense signal 274. If the crossing of the P-wave sensing threshold isconfirmed to be a true P-wave signal by analyzer 268, the pace timingand control module 270 controls pulse generator 202 to deliver a pacingpulse upon expiration of the AV pacing escape interval.

In some cases, the P-waves and T-waves are indistinguishable from eachother on the raw unfiltered EGM signal 265 and filtered EGM signals 266and 267 based on frequency, amplitude, morphology and timing. Ifanalyzer 268 determines that P-waves and T-waves are indistinguishable,the P-wave detector 262 may be temporarily disabled and/or pace timingmodule 270 may be disabled from receiving or using P-sense signals 272for the purposes of setting the ventricular pacing escape intervaltimer. The RV pacemaker 14 operates in a single chamber ventricularpacing and sensing mode until analyzer 268 determines that P-waves andT-waves are distinguishable again.

The techniques disclosed herein are described in the context of RVpacemaker 14 having a NF event detector 222 shown in FIG. 3 fordetecting NF R-waves (which may correspond to R-wave detector 252 inFIG. 4A) and a FF event detector 224 shown in FIG. 3 for detecting FFP-waves (which may correspond to P-wave detector 262 in FIG. 4A). It isunderstood that the disclosed techniques described in the context of aRV pacemaker implementation may be adapted for use in RA pacemaker 12.In such embodiments, the NF event detector 222 is configured to detectNF P-waves and the FF event detector 224 is configured to detect FFR-waves (and optionally FF T-waves). In this case, R-wave detector 252may correspond to the FF event detector 224 and may have an adjustablefilter 254 that is automatically adjusted to increase separation of FFR-waves from NF P-waves and FF T-waves in time, amplitude and/ormorphology in the filtered cardiac electrical signal.

In the examples described below in conjunction with FIGS. 5A, 5B and 5C,cardiac event sensing thresholds are described based on detecting FFP-waves from a cardiac electrical signal received by RV pacemaker 14. InRA pacemaker implementations, the relative amplitudes of these cardiacevent sensing thresholds may change based on the expected relativeamplitudes of the NF P-wave, FF R-wave and FF T-wave in a filteredcardiac electrical signal produced by the RA pacemaker FF event detector224.

FIG. 4B is a depiction of raw unfiltered EGM signals 280, 282 and 284received by P-wave detector 262. The raw unfiltered EGM signals 280,282, and 284 may correspond to raw, unfiltered EGM signal 265 shown inFIG. 4A passed by pre-filter 251 after wideband filtering to remove DCoffset and high frequency noise. The three raw, unfiltered EGM signals280, 282, and 284 are acquired using electrodes positioned at threedifferent inter-electrode spacings. Signal 280 is acquired using aninter-electrode spacing of 131 mm. Signal 282 is acquired using aninter-electrode spacing of 100 mm, and signal 284 is acquired using aninter-electrode spacing of 60 mm in this example. The greaterinter-electrode spacing may be achieved using a sensing extension, e.g.,as shown in FIG. 2B.

As observed in FIG. 4B, the amplitude and morphology of the P-waves 286a, 286 b and 296 c may change substantially with increasinginter-electrode distance. The R-waves 288 a, 288 b and 288 c, andT-waves 290 a, 290 b, and 290 c do not change as substantially withinter-electrode distance in the examples shown. The effect ofinter-electrode spacing on the P-wave and the relative differencesbetween P-waves, R-waves T-waves may vary with implant position ofelectrodes 162 and 164 as well as inter-electrode spacing.

In the example shown, the maximum peak amplitude and maximum slope ofP-waves 286 a, 286 b, and 286 c are observed to increase with increasinginter-electrode spacing because the proximal return electrode 162 may bepositioned closer to atrial tissue. The increased inter-electrodespacing may therefore be used to acquire P-waves having a higheramplitude that is more easily detected by a P-wave sensing thresholdcrossing applied by P-wave detector analyzer 268. The higher amplitudeof P-wave 286 a, for example, is still easily distinguishable from theR-wave amplitude, e.g., by using a P-wave sensing threshold set wellabove EGM baseline variation but lower than the R-wave peak amplitude asdescribed in greater detail below. In some cases, analyzer 268 mayfurther apply an intermediate T-wave sensing threshold greater than theP-wave sensing threshold and less than the R-wave sensing threshold todistinguish the P-wave 286 a, 286 b or 286 c from the T-wave 290 a, 290b, or 290 c. In other cases, the enhanced peak amplitude of P-wave 286 aand high slew rate or other distinct morphology features of the P-wave286 a may enable sensing of P-wave 286 a by analyzer 268 withoutrequiring an intermediate T-wave sensing threshold.

FIG. 4C is a depiction of filtered, rectified EGM signals 280′, 282′,and 284′ produced after filtering and rectifying the respectiveunfiltered EGM signals 280, 282 and 284 of FIG. 4B by P-wave detector262. Signals 280′, 282′ and 284′ are filtered by a 20 Hz highpass filterin this example, resulting in significant attenuation of T-waves 296 a,296 b and 296 c compared to respective raw, unfiltered T-waves 290 a,290 b and 290 c shown in FIG. 4B. As described below, in some examplessensing of P-waves 292 a, 292 b or 292 c from the filtered EGM signal266 received by analyzer 268 may be confirmed when a T-wave 290 a, 290 bor 290 c can be identified from the raw, unfiltered EGM signal 265(e.g., T-wave 290 a, 290 b, or 290 c of raw, unfiltered EGM signals 282,284 or 286, respectively) that is not coincident with a P-sense signal272.

In other examples, the T-wave may still be present in the filtered EGMsignal 266 with an amplitude that interferes with amplitude-based P-wavesensing in which case additional P-wave sensing criteria may be appliedby analyzer 268 for sensing P-waves. Additional P-wave sensing criteriamay be based on the higher slope (i.e., slew rate), narrow signal widthor other waveform morphology differences of the P-wave 292 a, 292 b or292 c compared to the lower slope and higher signal width of the T-wave296 a, 296 b or 296 c.

P-wave detector analyzer 268 may additionally or alternatively apply aT-wave sensing window 298, also referred to herein as a “T-wave window,”for confirming a P-wave sensing threshold crossing as a P-wave. T-wavewindow 298 may be applied after a post-sense blanking period 295following an R-sense signal 258 produced by R-wave detector 252 (or upondetecting an R-wave sensing threshold crossing of filtered EGM signal266 by P-wave detector analyzer 268 as described below in conjunctionwith FIG. 5B). Post-sense blanking period 295 may be applied by R-wavedetector 252 (or P-wave detector 262) to avoid double-sensing of R-wave294 a, 294 b or 294 c. A maximum peak amplitude may be determined duringblanking period 295 for setting the starting value of an auto-adjustingR-wave sensing threshold used by R-wave detector 252 and/or P-wavedetector 262.

The T-wave window 298 may be applied to exclude P-wave sensing thresholdcrossings during T-wave window 298 from being sensed as P-waves in someexamples. As described in greater detail below, T-wave window 298 may beadjusted to provide temporal discrimination of P-waves 292 a-c fromT-waves 296 a-c, respectively. T-wave sensing window 298 may be adjustedin response to heart rate changes, in response to changes between aventricular paced rhythm and a sensed intrinsic ventricular rhythm, andas needed to provide reliable P-wave sensing and discrimination fromT-waves.

FIGS. 5A, 5B, and 5C are diagrams of a P-wave, R-wave and T-wave of afiltered EGM signal 266 after rectification by analyzer 268. Varioussensing thresholds 304, 306 and 308 used by the sensing and pacingcontrol circuitry shown in FIG. 4A for sensing P-waves and controlling apacing escape interval timer are shown. FIG. 5A is a conceptual diagramof a P-wave 302 of filtered ventricular EGM signal 266 afterrectification by analyzer 268 of FIG. 4A. The analyzer 268 of P-wavedetector 262 may apply at least two sensing thresholds 304 and 306 tothe amplitude of EGM signal 266 and may apply a third intermediatethreshold 308. The first sensing threshold 304 is a relatively lower,P-wave sensing threshold and enables relatively low amplitude P-wave 302to be sensed. Sensing module 204 may set the P-wave sensing threshold304 to a value that is expected to be less than a maximum peak amplitudeof P-waves of the EGM signal 266, such as P-wave 302.

As the filtered EGM signal 266 begins to rise from a baseline, it willcross the lowest amplitude P-wave sensing threshold 304 first. Thefiltered EGM signal 266 may continue to increase to cross the second,R-wave sensing threshold 306. The second, R-wave sensing threshold 306is greater than the P-wave sensing threshold 304. An intermediate,T-wave sensing threshold 308 may be defined that is greater than theP-wave sensing threshold 304 and less than the R-wave sensing threshold306.

In some examples, the P-wave detector filter 264 is tuned to eliminateT-waves from the EGM signal 266 or reduce the maximum T-wave signalamplitude to be consistently less than P-wave sensing threshold 304. Insuch cases, the intermediate threshold 308 is not required. In otherexamples, P-wave detector filter 264 is tuned to enhance or maximizeT-wave amplitude so that it is consistently greater than the amplitudeof P-wave 302. The intermediate, T-wave sensing threshold 308 may bedefined to discriminate between the P-wave 302 and T-waves havingconsistently higher amplitude.

When the filtered EGM signal 266 crosses the P-wave sensing threshold304 at 312, it is unknown whether the EGM signal 266 will continue toincrease in amplitude and cross the R-wave sensing threshold 306 orT-wave sensing threshold 308. The early increasing portion of the EGMsignal 266 could be a P-wave, T-wave or R-wave. The P-wave sensingthreshold crossing alone is not enough evidence to confirm P-wave 302.

If the EGM signal 266 crosses the P-wave sensing threshold 304 outsideof any atrial blanking period, atrial sensing refractory period, orT-wave window, the threshold crossing at 312 is preliminarily determinedto be a P-wave signal. A P-sense signal 272 may be produced by P-wavedetector 262 and used by pace timing and control 270 to start aventricular pacing escape interval set equal to the target AV intervalat block 320. In one example, if the EGM signal 266 does not cross theR-wave sensing threshold 306 or the T-wave sensing threshold 308 (ifused) during a predetermined time limit 310, the P-wave thresholdcrossing at 312 is determined to be evidence of a P-wave. The AV pacingescape interval timer started upon P-wave threshold crossing 312 isallowed to continue without adjustment at block 322.

In some examples, EGM signal 266 is determined to cross the higherR-wave sensing threshold 306 or T-wave sensing threshold by determiningthe maximum peak amplitude of the rectified filtered EGM signal 266after the first threshold crossing 312 and comparing the maximum peakamplitude to the respective higher threshold 308 or 306. If the maximumpeak amplitude of the signal 302 after crossing the P-wave sensingthreshold 304 is not greater than at least one of the higher thresholds306 or 308, the threshold crossing 312 is confirmed to be a P-wave andthe AV pacing escape interval timer continues to run.

If the AV escape interval expires a ventricular pacing pulse isdelivered at the target AV interval. In other examples, as describedbelow, P-wave detector 262 may perform additional signal analysis toconfirm that the P-wave threshold crossing is a P-wave during the AVinterval. The cancel P-sense signal 274 may be produced by P-wavedetector 262 if additional signal analysis fails to confirm the P-wave.

The time limit 310 may be set by the sensing module 204 to a nominalvalue, e.g., 120 ms or based on an expected cardiac event width or slopeof the filtered cardiac electrical signal, e.g., an R-wave width, R-waveslope, T-wave width, or T-wave slope, such that if the EGM signalcrossing of the P-wave sensing threshold is actually an R-wave orT-wave, a higher R-wave sensing threshold or T-wave sensing thresholdwill be crossed within the time limit following the P-wave sensingthreshold crossing. The time limit is set shorter than an expected P-Rinterval to reduce the likelihood of two different events, e.g., oneP-wave and one R-wave, being sensed within the time limit.

For example if time limit 310 is set to at least half the expectedT-wave width or at least half of the expected R-wave width of thefiltered cardiac electrical signal, e.g., up to 80 ms, the EGM signal266 will cross the respective T-wave sensing threshold 308 or R-wavesensing threshold 306 within the time limit 310 if the rising amplitudeof the EGM signal is actually due to a T-wave or R-wave instead ofP-wave 302. Time limit 310 may be established by analyzer 268 of P-wavedetector 262 by determining the lowest slope or the widest signal widthof a T-wave or R-wave in the filtered EGM signal 266. Alternatively timelimit 310 is a nominal value stored in RV pacemaker memory 210, whichmay be based on clinical data and any expected delays in the filteredcardiac electrical signal.

FIG. 5B is a conceptual diagram of an R-wave 330 of a ventricular EGMsignal 266. When EGM signal 266 crosses the P-wave sensing threshold 304at time 332, the escape interval timer is set to an AV interval at block334. If the EGM signal 266 crosses the R-wave amplitude 306 within thetime limit 310, the escape interval timer started at block 334 isadjusted to a VV interval at block 336 in response to the R-wave sensingthreshold crossing.

In some examples, the P-wave detector analyzer 268 may set the R-wavesensing threshold 306. Alternatively, the separate R-wave detector 252(shown in FIG. 4A) produces an R-sense signal 258 in response to anR-wave 330, which causes the pace timing and control module 270 to resetthe pacing escape interval timer to the VV interval at block 336,effectively cancelling the relatively shorter AV interval started atblock 334.

FIG. 5C is a conceptual diagram of a T-wave 340 of the filtered EGMsignal 266. In some examples, P-wave detector filter 264 is adjusted tominimize the T-wave amplitude in the filtered EGM signal 266 such thatonly a P-wave sensing threshold 304 and R-wave sensing threshold 306 areused for controlling the ventricular escape interval as described inconjunction with FIGS. 5A and 5B. In other examples, the T-waveamplitude may be higher than the P-wave amplitude in the filtered EGMsignal 266. In this case, the T-wave threshold 308, intermediate theP-wave amplitude 304 and R-wave amplitude 306, is used to discriminatebetween P-waves 302 and T-waves 340. The amplitude of T-wave 340 may beintentionally increased through adjusting the center frequency andbandwidth of filter 264 to provide amplitude discrimination betweenP-waves 302 and T-waves 340.

When EGM signal 266 crosses the P-wave sensing threshold 304 at time342, the escape interval timer is set to an AV interval at block 344. Ifthe EGM signal 266 crosses the intermediate T-wave amplitude 308 withinthe time limit 310, the AV escape interval is cancelled at block 346without setting a new pacing escape interval. If the EGM signal 266 doesnot cross the R-wave sensing threshold 306, no change is made at block348, i.e., the escape interval remains cancelled and no new escapeinterval is started. In this way, the T-wave 340 is ignored for thepurposes of starting an escape interval and detection of the T-wavethreshold crossing with no R-wave threshold crossing within time limit310 cancels the AV interval started upon the P-wave threshold crossing.If the EGM signal 266 crosses R-wave threshold 306 during time limit 310after crossing T-wave threshold 308, a pacing escape interval set to adesired VV interval may be started.

Alternatively, instead of waiting for another threshold crossing, amaximum peak amplitude of the filtered EGM signal 266 (afterrectification) is determined in response to the P-wave sensing thresholdcrossing 342. If the maximum amplitude of the signal peak occurringafter the P-wave sensing threshold crossing 304 is greater than theR-wave sensing threshold 306, the AV pacing escape interval is changedto a VV pacing escape interval. If the maximum peak amplitude is greaterthan the T-wave sensing threshold 308, when used as an intermediatethreshold, but not greater than the R-wave sensing threshold 306, the AVpacing escape interval is canceled.

In the examples of FIGS. 5A, 5B, and 5C the P-wave sensing threshold304, T-wave sensing threshold 308 and R-wave sensing threshold 306 areeach shown as constant values. It is contemplated that a cardiac eventsensing threshold is an auto-adjusting threshold that has a startingthreshold amplitude that decays at one or more decay rates to a sensingfloor. In some examples, the cardiac event sensing thresholds describedherein may be automatically adjusted by the sensing module 204 based onmaximum peak cardiac event amplitude determined during a post-senseblanking interval and optionally using one or more decay rates andintervals and/or step-drop times.

In the example described above, the pace timing and control module 270responds to a P-wave threshold crossing, T-wave threshold crossing andR-wave threshold crossing as it occurs by starting an AV escapeinterval, cancelling the AV escape interval or changing to a VV escapeinterval respectively. In other examples, the pace timing and controlmodule 270 may wait for the time limit 310 to expire before respondingto a threshold crossing and/or P-wave detector 262 may wait for timelimit 310 to expire before producing the P-sense signal 272. If only aP-wave sensing threshold crossing occurs during time limit 310, theP-sense signal 272 is produced at the expiration of time limit 310. Thepace timing and control module 270 starts the escape interval timer setto the target AV interval (less time limit 310) in order to deliver theventricular pacing pulse at the target AV interval after P-wave 302. Ifthe R-wave sensing threshold 306 is crossed before time limit 310expires, pace timing and control module 270 starts a VV interval atblock 336 at the expiration of the time limit 310. If the intermediateT-wave sensing threshold 308 is crossed during time limit 310 but R-wavesensing threshold 306 is not crossed, no P-sense signal 272 is producedand no escape interval is started.

The initial crossing of the P-wave sensing threshold 304 is invalidatedas a true P-wave signal if the EGM signal amplitude reaches the higherT-wave sensing threshold 308 or R-wave sensing threshold 306 within timelimit 310. The P-wave detector 262 may produce the Cancel P-sense signal274 in response to a crossing of the T-wave sensing threshold 308 orR-wave sensing threshold 306 to cause pace timing and control module 270to cancel an AV interval started at block 344.

In some cases, it may be possible to adjust filter 264 to cause theamplitude of T-wave 340 to be greater than the amplitude of R-wave 330,depending on the location of electrodes 162 and 164. The T-wavethreshold would be the highest threshold and the R-wave threshold wouldbe the intermediate threshold in this case. If the EGM signal 266crosses the P-wave threshold, the AV interval is started. If theintermediate R-wave threshold is crossed, the AV interval is changed toa VV interval. If the highest, T-wave threshold is crossed, the VVinterval is cancelled.

FIG. 6 is a flow chart 300 of a method for controlling ventricularpacing pulse delivery using FF P-wave sensing by RV pacemaker 14.Methods described in conjunction with flow chart 300 and other flowcharts presented herein may be implemented in a computer-readable mediumthat includes instructions for causing a programmable processor to carryout the methods described. The instructions may be implemented as one ormore software modules, which may be executed by themselves or incombination with other software.

At block 350, a P-wave threshold crossing is detected by P-wave detector262. It is recognized that the P-wave threshold crossing detected atblock 350 used for setting an AV pacing escape interval may be requiredto occur outside an atrial blanking interval and outside apost-ventricular atrial refractory period.

Furthermore, a T-wave window may be established by P-wave detector 262for discriminating between P-waves and T-waves based on relative timingfrom an R-wave. After receiving an R-sense signal 258, P-wave detector262 may start a T-wave window during which any P-wave sensing thresholdcrossings are ignored as most likely being T-waves. As such, if theP-wave sensing threshold crossing detected at block 350 is during aT-wave window as determined at block 351, the P-wave detector 262 mayreturn to block 350 to wait for the next P-wave threshold crossing. TheT-wave window may begin upon receiving the R-sense signal 258 (or upondelivering a ventricular pacing pulse) and expire at a time intervalafter the R-sense signal 258 (or ventricular pacing pulse) that isexpected to encompass a T-wave but not the P-wave of the next cardiaccycle. The T-wave window may be approximately 300 to 600 ms long in someexamples, and may extend at least approximately 500 ms after the R-waveduring resting heart rates. The T-wave window may be adjusted based onheart rate and/or based on whether the T-wave window is set in responseto an R-sense signal 258 or in response to a ventricular pacing pulse.

If a non-refractory P-wave threshold crossing is detected outside aT-wave window (“yes” branch of block 351), the P-wave detector 262starts the time limit 310 (shown in FIGS. 5A-5C) at block 352. TheP-wave detector 262 may produce a P-sense signal 272, and pace timingand control module 270 responds to the P-sense signal by setting theventricular pacing escape interval timer to an AV interval at block 354.If a T-wave sensing threshold crossing is detected at block 355 beforethe time limit 310 expires (block 356), P-wave detector 262 may producea Cancel P-sense signal 274 (FIG. 4A) causing pace timing and control270 to immediately cancel the pacing escape interval at block 362.

Alternatively, the pace timing and control module 270 may wait for thetime limit 310 to expire to determine if an R-wave sensing thresholdcrossing is detected at block 358. If the T-wave sensing threshold iscrossed (block 355), but the R-wave sensing threshold is not reached(block 358) before the time limit expires (block 360), the AV pacingescape interval started at block 354 is cancelled at block 362. It is tobe understood that in some examples, the P-wave detector filter 264eliminates or significantly attenuates T-wave signals so that the peakT-wave amplitude is significantly smaller than the peak P-waveamplitude. In this case, a T-wave sensing threshold may not be required.Pace timing and control module 270 may monitor only for an R-wavesensing threshold crossing at block 358 within the time limit 310 afterthe P-wave sensing threshold crossing.

If the R-wave sensing threshold is crossed within the time limit atblock 358, the pace timing and control module 270 changes the pacingescape interval timer from the AV interval set at block 354 to a VVinterval at block 364. The crossing of an R-wave sensing threshold maybe determined by R-wave detector 252 at block 358, resulting in anR-sense signal 258, or by P-wave detector 262 causing a Cancel P-sensesignal 274 to be produced.

As indicated above, rather than waiting for a T-wave threshold crossingat block 355 or an R-wave threshold crossing at block 358, the P-wavedetector 262 may determine a maximum peak amplitude of the filtered EGMsignal 266 after the P-wave threshold crossing and compare the maximumpeak amplitude to a T-wave threshold and/or R-wave threshold atrespective blocks 355 and 358.

During the VV pacing escape interval, the R-wave detector 252 and P-wavedetector 262 may continue to monitor for R-waves and P-waves at block368 by monitoring for a new P-wave sensing threshold crossing outsideany applicable atrial blanking or refractory periods. For example anatrial blanking interval and a post-ventricular atrial refractory periodmay be set after the R-wave sensing threshold crossing is detected atblock 358. If a new crossing of the P-wave sensing threshold is detectedat block 368, during the currently running escape interval but outsideany atrial blanking or refractory, and outside the T-wave window asdetermined at block 351, the pace timing and control module 270 restartsthe time limit at block 352. The ventricular pacing escape intervaltimer is reset to the target AV interval at block 354. The process ofwaiting to confirm the threshold crossing as a P-wave based on no highersensing threshold crossings within the time limit repeats.

If the AV pacing escape interval set at block 354 expires at block 366,the pace timing and control module 270 controls the pulse generator 202to deliver a ventricular pacing pulse at block 370. In some examples,the AV pacing escape interval is set up to a maximum AV pacing escapeinterval limit to prevent delivering a ventricular pacing pulse duringthe T-wave following a premature ventricular contraction (PVC). A PVCmay meet the P-wave sensing criteria and be falsely sensed as a P-wave,causing an AV pacing escape interval to be started. If the AV intervalis longer than a maximum limit, e.g., longer than 200 to 300 ms, theventricular pacing pulse may be delivered during the T-wave which can bearrhythmogenic in some patients.

If the VV pacing escape interval started at block 364 in response to acrossing of the R-wave sensing threshold expires at block 366, aventricular pacing pulse is delivered at block 370 by the pulsegenerator 202 under the control of pace timing and control module 270.

A ventricular pulse delivery causes pace timing and control 270 to resetthe escape interval timer to a VV pacing escape interval at block 372.The process returns to block 366 after starting the VV pacing escapeinterval to wait for the escape interval to expire while monitoring fora new P-wave sensing threshold crossing at block 368 during the escapeinterval but outside any relevant blanking or refractory window.

FIG. 7A is a flow chart 380 of a method for verifying a P-wave senseevent during an AV interval set in response to a P-wave sensingthreshold crossing. As described in conjunction with FIG. 6, P-wavedetector 262 detects a P-wave sensing threshold crossing and starts atime limit for sensing a next, higher sensing threshold. The pace timingand control module 270 sets the escape interval timer to the target AVpacing escape interval. If the time limit expires without sensing ahigher sensing threshold crossing, the AV interval will be running asindicated at block 382. During this AV interval, additional operationsmay be performed by P-wave detector 262 to verify that the P-wavesensing threshold crossing is a true P-wave. The process shown in FIG.7A may therefore be performed after the time limit 310 expires andbefore the AV interval expires to confirm that the signal is a P-wavebefore delivering a ventricular pacing pulse at block 370 of FIG. 6.

P-wave sensing criteria may be applied to the filtered EGM signal 266and/or the raw unfiltered EGM signal 265 at block 384. In one example, aP-wave analysis segment may be digitized and stored extending from theP-wave sensing threshold crossing (or a defined time interval or numberof sample points earlier) until a defined time interval after the P-wavesensing threshold crossing. The P-wave analysis segment may be set basedon an expected P-wave width for example. The signal during the P-waveanalysis segment is compared to P-wave sensing criteria at block 384.

P-wave sensing criteria may include a maximum and/or minimum signalwidth, signal slope, number of inflection points, frequency content orother signal feature threshold. One or more EGM signal features duringthe P-wave analysis segment, of either the filtered EGM signal 266 orthe raw unfiltered EGM signal 265, not meeting the P-wave sensingcriteria may disqualify the P-wave sensing threshold crossing as a trueP-wave.

In other examples, P-wave sensing criteria may include time intervalranges relative to a previous T-wave or R-wave sensing thresholdcrossing, or relative to an R-sense signal 258, to discriminate betweenthe expected timing of a P-wave (after a preceding R-wave or a T-wave)and the expected timing of a T-wave (after the preceding R-wave). Forexample, if the P-wave sensing threshold crossing occurs within a T-wavewindow set relative to an R-wave sense event signal, this timingevidence may be used to disqualify the P-wave sensing threshold crossingas a true P-wave. If the P-wave sensing threshold crossing occursoutside the T-wave sensing window, the crossing of the P-wave sensingthreshold may be confirmed as a P-wave as long as the EGM signal meetsany other P-wave sensing criteria requirements.

In still other examples, P-wave sensing criteria applied at block 384may include criteria applied to a comparison of the P-wave signalanalysis segment to an overall waveform morphology template. Samplepoints of the filtered, digitized EGM signal acquired at regularsampling intervals across the entire signal analysis segment may becompared to a previously stored known P-wave morphology template and/ora previously stored known T-wave morphology template in order to confirma high correlation of the unknown EGM signal with the P-wave morphologytemplate and/or a low correlation with the T-wave morphology template.If the signal analysis segment is highly correlated with a T-wavemorphology template or is poorly correlated with a P-wave morphologytemplate, the P-wave threshold crossing may be disqualified as a trueP-wave.

If a P-wave sense event is verified at block 386 based on the comparisonof the EGM signal to other P-wave sensing criteria, pace timing andcontrol module 270 waits for the AV pacing escape interval to expire atblock 388. It is recognized that an R-wave sensed during the AV intervalwill cause the escape interval timer to be reset to a VV interval.Otherwise, a ventricular pacing pulse will be delivered upon expirationof the AV pacing escape interval following the verified P-wave.

If a P-wave sense event is not verified in response to the comparisonmade at block 384, the AV pacing escape interval is cancelled at block390 prior to its expiration. At block 392, the control module 206 of RVpacemaker 14 may determine if a P-wave sense event has been verifiedwithin the previous n seconds, minutes, hours or other predeterminedtime interval.

In one example, if a P-wave has not been sensed in the past 20 seconds,the P-wave sensing criteria used by the analyzer 268 of P-wave detector262 is checked and updated if needed at block 394. A time interval up toone minute, for example, may be set after which an absence of a verifiedP-wave will trigger the P-wave sensing criteria check at block 394. Instill other examples, the time interval after which the P-wave sensingcriteria is checked if no P-waves have been verified during the timeinterval may be a progressively increasing time interval. The first timeinterval may be relative short, e.g., 30 seconds and the next timeinterval may be doubled or increased by a predetermined increment fromthe first time interval. To illustrate, as long as at least one oranother required minimum number of P-waves are verified within a currenttime interval, the next time interval is doubled such that the timeinterval series may include intervals of 30 seconds, 60 seconds, 2minutes, 4 minutes and so on up to a maximum time interval of one hourfor example. After the maximum time interval is reached, the maximumtime interval is repeatedly used at block 392. If a required number ofP-waves are not sensed during the time interval, P-wave sensing criteriaare checked and updated if needed. The time interval series begins againat the first, shortest time interval after an update of the P-wavesensing criteria. Processing power and time are conserved by usingrelatively longer or progressively increasing time intervals betweenchecking and updating P-wave sensing criteria.

If a P-wave has been confirmed recently based on the P-wave sensingcriteria applied at block 384 being satisfied within the N-secondinterval (block 392), the pace timing and control module 270 returns toblock 382 to wait for the next time limit to expire during the nextrunning AV interval. The P-wave sensing criteria used at block 384 isdeemed valid if a minimum number of P-waves have been confirmed duringthe most recent n-seconds.

If a P-wave has not been confirmed within the n-second time interval asdetermined at block 392, the P-wave sensing criteria used at block 384to verify a true P-wave may need updating. A change in RV pacemakerposition, myocardial substrate, a prescription medication, heart rate,patient position, patient activity or other change or condition mayalter the EGM signal such that the P-wave sensing criteria requiresupdating. Accordingly, the P-wave sensing criteria are checked andupdated if needed at block 394 as further described in conjunction withFIG. 8.

FIG. 7B is a depiction of a raw, unfiltered EGM signal 450, a filtered,rectified EGM signal 460, a differential EGM signal 470 and anintegrated EGM signal 480. Raw, unfiltered EGM signal 450 corresponds tothe wideband pre-filtered EGM signal 265 received by P-wave detector 262in FIG. 4A and includes a P-wave 452, R-wave 454, and T-wave 456.Filtered, rectified EGM signal 460 corresponds to the filtered EGMsignal 266 after rectification by P-wave detector analyzer 268 andincludes P-wave 462, R-wave 464 and T-wave 466.

Analyzer 268 may include a differentiator to produce a differential EGMsignal 470, which is the differential signal of filtered, rectified EGMsignal 460 in this example. In other examples, either one or both ofunfiltered EGM signal 450 and filtered EGM signal 460 are differentiatedto produce a differential signal used to discriminate P-waves 472 fromR-waves 474 and T-waves 476. The T-wave 476 of the differential signalis largely attenuated in the differential signal 470 compared to P-wave472.

Analyzer 268 may further include an integrator to produce an integratedEGM signal 480. In the example shown, integrated EGM signal 480 is theintegrated signal of filtered, rectified EGM signal 460. Analyzer 268may be configured to produce an integrated signal from one or both ofraw, unfiltered EGM signal 450 and filtered EGM signal 460, beforeand/or after rectification. In the integrated signal 480, P-wave 482 islargely attenuated compared to R-wave 484 and T-wave 486.

P-wave detector 262 may be configured to apply other P-sense criteria atblock 384 of FIG. 7A to one or more of the unfiltered EGM signal 450,filtered rectified EGM signal 460, differential signal 470 and/orintegrated signal 480.

In some examples, P-wave detector 262 is configured to determine themaximum peak amplitude of the filtered rectified EGM signal 460 inresponse to a P-wave sensing threshold crossing and compare the maximumpeak amplitude to T-wave and/or R-wave sensing thresholds in parallel toapplying one or more additional P-wave sensing criteria to theunfiltered EGM signal 450, filtered rectified EGM signal 460,differential signal 470 and/or integrated signal 480.

For example, the differential signal 470 may be determined in parallelto the filtered, rectified EGM signal 460. This differential signal 470filters the T-wave 476. The P-wave 462 of filtered, rectified EGM signal460 may cross both a P-wave and T-wave sensing threshold, but only theP-wave 472 of differential signal 470 will cross the P-wave sensingthreshold on the differential signal. In this way, a signal that crossesboth the P-wave and T-wave sensing thresholds in the filtered, rectifiedsignal 460 can identified as a T-wave if the differential signal 470does not cross a P-wave sensing threshold and identified as a P-wave ifthe differential signal 470 does cross a P-wave sensing threshold.

In another example, a slew rate may be determined from unfiltered EGMsignal 450 or from differential signal 470 and compared to a P-wave slewrate threshold by analyzer 268 to discriminate a P-wave sensingthreshold crossing from a T-wave. The T-wave 476 of differential signal470 is expected to be significantly attenuated compared to P-wave 472.The high slew rate of the P-wave 472 in differential signal 470 may be astrong discriminator between P-wave 472 and T-waves 476. If the slewrate of raw, unfiltered EGM signal 450 or of differential signal 470,which may be determined using signal sample points before and/or afterthe P-wave sensing threshold crossing of filtered EGM signal 460, isgreater than a P-wave slew rate threshold, the P-wave sensing thresholdcrossing is confirmed as a sensed P-wave as long as an R-wave sensingthreshold is not crossed within a time limit.

The polarity of the differential signal determined from a non-rectifiedsignal may be useful for discriminating P-waves from R-waves in somecases. The P-wave and R-wave may have opposite polarities as shown inraw unfiltered EGM signal 450. When the absolute peak amplitudes andslopes of the P-wave and R-wave approach each other, the P-wave of thedifferential signal determined from raw unfiltered signal 450 may have apolarity that is distinct from the R-wave.

In another example, the maximum peak amplitude and/or signal width ofthe integrated signal 480 may be determined after the filtered EGMsignal 460 crosses the P-wave sensing threshold. The maximum peakamplitude and/or signal width of the integrated signal 480 may becompared to a maximum P-wave amplitude sensing threshold and/or maximumP-wave signal width sensing threshold. Since the T-wave 486 is increasedin amplitude and signal width compared to the P-wave 482 in theintegrated signal 480, a maximum peak amplitude and/or signal width ofintegrated signal 480 that exceeds the respective maximum P-waveamplitude or signal width sensing threshold invalidates the P-wavesensing threshold crossing of the filtered EGM signal as being a trueP-wave (and may be used to sense a T-wave 486 or R-wave 484 based ontiming relative to an R-sense signal 258).

When the P-wave 452 in the raw unfiltered signal 450 has a low slewrate, the P-wave detector filter 264 may need to be adjusted to a lowerlow-frequency cut-off, e.g., from 20 Hz to 10 Hz in order to avoidsignificant attenuation of the P-wave in filtered EGM signal 460. T-wave466 may be less attenuated in the filtered EGM signal 460 when thefilter bandpass is lowered. As a result, T-waves 466 may be oversensedas P-waves due to P-wave sensing threshold crossings by T-waves 466 whenthe P-wave 462 and T-wave 466 are similar in amplitude. In thissituation, the analysis of the differential signal 470 and/or integratedsignal 480 may enable identification and discrimination of the P-waves472 or 482 from T-waves 476 or 486, respectively.

The differential signal 470 and the integrated signal 480 shown ordescribed above need not be determined continuously, but may bedetermined over a predetermined time interval when the filtered EGMsignal 460 crosses the P-wave sensing threshold, e.g., over an intervalof approximately 100 ms to 200 ms. The criteria established fordetecting and confirming P-waves and distinguishing P-waves from T-wavesand R-waves in the methods described herein may be based on particularfeatures of any combination of the unfiltered EGM signal 450 received byP-wave detector analyzer 268, the filtered EGM signal 460, differentialsignal 470 and/or integrated signal 480, and may include rectifiedand/or unrectified signals of any of the foregoing signals.

In some examples, the alternate signals, such as differential signal 470and integrated signal 480, are determined for verifying P-wave sensingduring an AV interval on a beat-by-beat or less frequent basis.Additionally or alternatively, one or more alternate signals, e.g., rawunfiltered signal 450 or a differential or integrated signal thereof,differential signal 460 and/or integrated signal 470, are used onlyduring a process for rechecking and establishing P-wave sensing criteriaat block 394 of FIG. 7A. A feature, such as a threshold crossing, peakamplitude, slew rate, signal width or other signal feature of thealternate signal at a time of a first P-wave sensing threshold crossingby filtered EGM signal 460 may be compared to the same feature of thealternate signal at the time of the next P-wave sensing thresholdcrossing by the filtered EGM signal 460 to verify that both events areP-waves or to discriminate one crossing as being a P-wave and onecrossing as being a T-wave.

The alternate signal may be used to discriminate the P- and T-waves toenable sensing module 204 (and/or control module 206) to establishP-wave sensing criteria based only on filtered EGM signal 460 or anycombination of the filtered EGM signal 460 and/or one or more alternatesignal as listed above. Established P-wave sensing criteria may then beapplied to the filtered EGM signal 460 (and/or alternate EGM signal)during P-wave sensing for atrial-ventricular synchronous pacing withoutrequiring beat-to-beat determination of a differential and/or integratedsignal.

FIG. 8 is a flow chart 400 of a method for establishing and updatingP-wave sensing criteria. The process shown in FIG. 8 may be performed bysensing module 204 and/or control module 206, e.g., by executinginstructions stored in memory 210. In the techniques described inconjunction with FIG. 8 and other flow charts presented herein, analysisof the filtered EGM signal, an unfiltered EGM signal, and/or alternateEGM signal and adjustments made to the P-wave detector filter 264 and/ora T-wave sensing window based on the analysis may be executed by sensingmodule 204, under the control of control module 206 in some examples, orcooperatively by sensing module 204 and control module 206.

At block 401, the control module 206 determines if it is time to checkP-wave sensing to verify reliable P-wave sensing and update P-wavesensing criteria if needed. The process may be started at block 401manually any time using external device 20, automatically upon initialimplantation of the RV pacemaker 14, and/or repeated automatically on ascheduled periodic basis. In other examples, the process is performedafter a P-wave has not been verified for a predetermined time intervalas described in conjunction with FIG. 7. In still other examples,control module 206 determines that a re-check is needed at block 401 inresponse to detecting a posture change, a change in patient activitylevel, a change in heart rate, a change between sustained R-wave sensingand sustained ventricular pacing or other change or condition that mayalter the amplitude or morphology of the P-wave signal, which may bedetected using sensors 212.

When control module 206 determines that it is time to check P-wavesensing, sensing module 204 starts the process of checking P-wavesensing by detecting a crossing of the P-wave sensing threshold by thefiltered EGM signal 266 at block 402. The P-wave sensing thresholddetected at block 402 is a non-blanking, non-refractory sensingthreshold crossing and may be required to be outside a T-wave window. Afirst waveform of the filtered EGM signal 266 is stored over a firstanalysis window at block 404 in response to the threshold crossing. Forexample, the first analysis window may be started at the P-wave sensingthreshold crossing, or a predetermined interval or number of samplepoints prior to the sensing threshold crossing, and extend apredetermined interval or number of sample points after the sensingthreshold crossing. A maximum peak amplitude or peak-to-peak amplitudedifference during the analysis window is compared to an R-wave thresholdat block 406. If the peak amplitude or peak-to-peak difference reachesan R-wave threshold, the crossing of the P-wave sensing threshold isevidence of an R-wave instead of a P-wave. The process returns to block402 to wait for the next P-wave sensing threshold crossing.

If the peak amplitude or peak-to-peak amplitude difference is less thanthe R-wave threshold at block 406, the sensing module 204 waits for thenext R-sense signal 258 (or a ventricular pacing pulse when an R-sensesignal is not received) at block 408. The P-wave threshold crossing atblock 402 may be a P-wave or a T-wave but is presumed not to be anR-wave. After the next R-sense signal 258 (or ventricular pacing pulse),the next P-wave sensing threshold crossing is detected at block 410.

In the example shown, an analysis of the EGM signal is performed tocheck P-wave sensing criteria regardless of whether a T-wave sensingthreshold is crossed. In some cases, a T-wave sensing threshold is notbeing used. In other examples, if a T-wave sensing threshold has beenestablished that is greater than the P-wave sensing threshold, theP-wave detector 262 may set a time limit to determine if the T-wavesensing threshold is crossed within the time limit after the P-wavesensing threshold crossing at block 410. Checking of the P-wave sensingcriteria may not be required as long as the EGM signal does not crossthe T-wave sensing threshold during the first analysis window but doescross the T-wave sensing threshold within the time limit following thenext P-wave sensing threshold crossing after the R-sense signal. TheP-wave before the R-sense signal and the T-wave after the R-sense signalare discriminated based on amplitude. The control module may return tothe process shown in FIG. 6 for controlling ventricular pacing usingfar-field P-wave sensing.

In other examples, the P-wave sensing criteria are checked by continuingto block 412 after detecting the next P-wave sensing threshold crossingat block 410 after the R-sense signal. At block 412, a second waveformof the filtered EGM signal 266 is stored over a second analysis windowin response to the next P-wave sensing threshold crossing. The secondanalysis window may be analogous to the first analysis window in timingand duration. In this way, the P-wave detector analyzer 268 acquires awaveform of a sensed signal occurring before an R-sense signal and awaveform of a sensed signal after the R-sense signal. Further analysisof these waveforms will reveal whether P-waves are being properly sensedand discriminated from T-waves.

P-wave detector 262 determines if the next crossing of the P-wavesensing threshold detected at block 410 occurred within a T-wave sensingwindow after the R-sense signal at block 414. A T-wave sensing windowmay be defined to extend from the R-sense signal (or a ventricularpacing pulse) to a time point after the R-sense signal so that it isexpected to encompass the T-wave but terminate prior to the nextanticipated P-wave. The T-wave sensing window may be started when anR-wave sensing threshold crossing is detected by R-wave detector 252 orupon receiving the R-sense signal 258 by P-wave detector 262. The T-wavesensing window may also be started in response to delivering aventricular pacing pulse when an R-wave is not sensed at block 408. TheT-wave sensing window may be set to different intervals depending on theventricular rate and depending on whether the initiating event is anR-sense signal or a ventricular pacing pulse.

If the next P-wave threshold crossing detected at block 410 is within aT-wave sensing window, as determined at block 414, the first and secondwaveforms stored during the first analysis window and the secondanalysis window are compared at block 420. The comparison made at block420 may include a sample-by-sample morphology comparison of the twowaveforms, wavelet morphology comparison, and/or determining one or moreanalogous features of the stored waveforms and comparing the analogousfeatures to each other. Analogous features that may be determined andcompared include, but are not limited to, peak amplitude, peak slope,signal width, number of peaks, a time interval from the P-wave sensingthreshold crossing to a signal peak, time interval from the thresholdcrossing to a peak positive slope, time interval from the thresholdcrossing to a peak negative slope, time interval from the thresholdcrossing to another fiducial point or other time intervals between otherfiducial points.

The two waveforms may be determined to match each other at block 422 ifeach comparison made at block 420 results in a difference that is lessthan predetermined matching criteria, e.g., a difference of less than20% between each feature being compared. The particular matchingcriteria or matching thresholds used will depend on the morphology orfeature comparisons being made.

If the two waveforms do not match as determined at block 422, based onat least one or more features not meeting the matching criteria, thefirst waveform preceding the R-sense signal is assumed to be a P-waveand the second waveform falling into the T-wave sensing window and notmatching the first waveform is assumed to be a T-wave. The P-wave isdistinguishable from the T-wave following the R-sense signal based onboth time and morphology indicating reliable P-wave sensing anddiscrimination from T-waves.

P-wave sensing is enabled at block 442. P-wave sensing criteria are setat block 444 by sensing module 204 based on the one or more featuresdetermined to be distinct during the comparison of the first and secondwaveforms at block 420. For example, a threshold or range may be definedthat is characteristic and inclusive of a given P-wave feature andexclusive of the analogous T-wave feature. To illustrate, a maximumP-wave signal width threshold may be defined as being 30 ms based on ameasured P-wave signal width of 20 ms and a measured T-wave signal widthof 40 ms during the comparison made at block 420. It is recognized thatcomparisons may be made between multiple P-waves and multiple T-waves toimprove the confidence of P-wave sensing criteria set at block 444. TheP-wave sensing criteria set at block 444 may be the same criteriapreviously used if the same criteria still discriminate well betweenP-waves and T-waves. Alternatively, the criteria set at block 444 may benew criteria established by sensing module 204 in response to thecomparing performed at block 420 and based on differences identifiedduring the comparing.

The “no” branch of block 422 as just described represents the situationof the P-wave and the T-wave being reliably distinguished based on thecomparison of the first and second waveforms not matching and the secondwaveform falling in the T-wave sensing window. In other cases, thesecond threshold crossing may be outside the T-wave sensing windowand/or the first and second waveforms may match. In these cases, bothwaveforms may be a P-wave (e.g., if the T-wave has been well-filteredfrom the EGM signal), both waveforms may be T-waves, or one waveform maybe a P-wave and one a T-wave but indistinguishable from each other basedon the current filtering and T-wave sensing window settings. The otherbranches of flow chart 400 handle these other situations as will now bedescribed.

If the next P-wave threshold crossing detected at block 410 is withinthe T-wave sensing window (“yes” branch of block 414) but the twowaveforms match at block 422, both waveforms could be P-waves.Alternatively, one may be a P-wave and one may be a T-wave but they areindistinct from each other in the filtered EGM signal 266. P-wavesensing by P-wave detector 262 may be unreliable under the currentconditions for use in setting AV pacing escape intervals. Accordingly,to determine if both waveforms are P-waves or one is a P-wave and one isa T-wave, the raw, unfiltered EGM signal 265 (or an alternate filteredEGM signal 267) is analyzed at block 423 to determine if T-waves arepresent in the unfiltered EGM signal and are not present in the filteredEGM signal 266. Peak amplitudes, morphology, timing relative to anR-sense signal or other features may be used to identify T-waves presentin the alternate filtered EGM signal 267 or unfiltered EGM signal 265.

If T-waves are identified in the alternate or unfiltered EGM signal thatare not present in the filtered EGM signal 266, e.g., not coinciding oroccurring at approximately the same time after an R-sense signal as oneof the first or next P-wave sensing threshold crossings detected atblocks 402 and 410, P-wave sensing is confirmed at block 424. Both thefirst waveform and the second waveform are determined to be P-waves whenT-waves can be identified from the unfiltered EGM signal 265 (or analternate filtered EGM signal 267) and are not present in the filteredEGM signal 266 from which the first and second waveforms were obtainedand not coinciding with either of the first or second waveforms. TheT-wave is optimally filtered from the filtered EGM signal 266. P-wavesensing is deemed reliable, however the T-wave sensing window may beadjusted at block 430 by sensing module 204 so that the second waveformthat is actually a P-wave does not fall in the T-wave sensing window.For example, the T-wave sensing window may be shortened at block 430.P-waves are determined to be reliably discriminated from T-waves atblock 432 based at least on the effective filtering of the T-wave fromthe filtered EGM signal 266 and may be further discriminated based ontiming by adjusting the T-wave sensing window at block 430. P-wavesensing is enabled at block 442 for use in synchronizing ventricularpacing with P-sense signals 272 using a target AV interval. P-wavesensing criteria are set at block 444 by sensing module 204 based on theP-wave features determined at block 420. These criteria may be used forconfirming a P-wave during an AV interval started in response to aP-wave threshold crossing. Setting P-wave sensing criteria may includesetting the P-wave sensing threshold based on an amplitude of the storedfirst and second waveforms.

The “yes” branch of block 424 just described is the situation that bothof the first and second waveforms before and after the R-sense signalare P-waves, but the second P-wave is within the T-wave sensing window.This situation may be corrected by adjusting the T-wave sensing window.T-waves are being properly filtered so filter adjustment is notrequired.

The “no” branch of block 424, however, addresses the situation where thesecond waveform is a T-wave that occurs within the properly set T-wavewindow but the T-wave and the P-wave morphologies are too similar to bedistinguishable based on waveform comparisons. In this case, the T-wavesensing window does not require adjusting, but adjustment of the P-wavedetector filter 264 by sensing module 204 may provide greaterdiscrimination between the P- and T-wave morphologies.

If T-waves are present in the unfiltered or alternate filtered EGMsignal 265 or 267 and correspond in time to the second waveform of thefiltered EGM signal 266 as determined at block 423, the two P-wavesensing threshold crossings of the filtered EGM signal 266 are not bothP-waves (“no” branch of block 424). In response to not confirming bothwaveforms as being P-waves, the bandpass of filter 264 may be adjustedto alter the waveform of the T-wave at block 426. As described above,the center frequency and/or bandpass of filter 264 may be adjusted tointentionally increase the amplitude of the T-wave, decrease theamplitude of the T-wave, or otherwise modify the T-wave morphology tomake the P-wave and T-wave distinct based on amplitude and/or waveformcomparisons. Multiple filter adjustments may be made until a comparisonof the P-wave and T-wave amplitudes, signal widths and/or othermorphology feature(s) results in non-matching waveforms.

After filter adjustments, P-wave and T-wave discrimination is confirmedat block 432. The P-wave and T-wave were already discriminated based onthe properly set T-wave sensing window so the result at block 432 willbe “yes” based on this time distinction alone. However, if the adjustedfilter frequency range provides further discrimination based onmorphology or amplitude, independent of the timing of the T-wave sensingwindow, the adjusted filter center frequency and bandwidth is selectedby the P-wave detector 262 for sensing P-waves for use in controllingescape intervals.

P-wave and T-wave discrimination determined at block 432 after filteradjustment may include elimination or significant attenuation of theT-wave, such that it no longer crosses the P-wave sensing threshold, oran increased T-wave amplitude that crosses a higher, T-wave sensingthreshold. As such, filter adjustment may provide amplitudediscrimination in addition to the timing discrimination provided by theT-wave sensing window.

Additionally or alternatively, the T-wave may be altered enough by abandpass filter adjustment that the P-wave and T-wave waveforms aredistinguishable based on morphology, slope or other waveform featuresother than peak amplitude. Filter adjustment may therefore providemorphology discrimination based on the overall wave shape or othersignal features other than peak amplitude.

As long as at least one of the timing, peak amplitude, and waveformmorphology provide P-wave and T-wave discrimination as determined atblock 432, P-wave sensing may be enabled at block 442 for setting AVpacing escape intervals. The discriminatory features identified at bock432 after filter adjustment may be used at block 444 to set P-wavesensing criteria that are used during AV intervals for confirming that aP-wave threshold crossing is a P-wave (e.g., for confirming a sensedP-wave at block 384 of FIG. 7).

Adjustment of the T-wave sensing window or the P-wave detector filter bysensing module 206 have now been described for addressing the situationof the second waveform being a P-wave but improperly falling into theT-wave sensing window (“yes” branch of block 424 leading to T-wavesensing window adjustment at block 430) and for the situation of thesecond waveform being a T-wave and properly falling into the T-wavesensing window) but having an indistinct morphology from the P-wave(“no” branch of block 424 leading to P-wave detector filter adjustmentat block 426). Now techniques will be described that deal with thesituation of the second waveform not falling into the T-wave sensingwindow, whether it is a true T-wave or not.

If the second waveform is not within the T-wave sensing window, “no”branch of block 414, the first and second waveforms are compared atblock 418. If the waveforms do not match (“no” branch of block 428), thesecond waveform may be a T-wave, but the T-wave sensing window may needadjusting. The non-matching waveforms indicate that the P-wave detectorfilter 264 does not necessarily need adjusting since the waveforms aredistinguishable based on the morphology. The T-wave sensing window mayneed to be adjusted at block 430, however, because the Q-T interval ofthe EGM signal may change over time, e.g., due to changes in heart rate,change in disease state or other factors. For example, the T-wavesensing window may be shortened by approximately 20 ms for everyincrease in heart rate of 10 beats per minute. The T-wave window may beadjusted by sensing module 204 at block 430 so that it would encompassthe second waveform. A different T-wave window may be set when aventricular pacing pulse is delivered than when an R-sense signal isreceived to account for differences in the Q-T interval on paced beatscompared to intrinsic beats.

Since the first and second waveforms do not match based on themorphology comparison at block 428, discrimination of the P-waves andT-waves is confirmed at block 432. The T-wave sensing window adjustmentmay provide additional discrimination of the P-waves and T-waves basedon timing. P-wave sensing for controlling pacing escape intervals isenabled by control module 206 at block 442. The P-wave sensing criteriamay be adjusted at block 444 by sensing module 204 as needed based onthe waveform features determined and compared at block 418.

Baseline P-wave and T-wave amplitude measurements and baseline T-Pinterval measurements may be determined and stored at block 444 aftersetting the P-sense criteria. As described in conjunction with FIG. 10A,the control module 206 may be configured in some examples to monitor anamplitude difference between P-waves and T-waves and/or a T-P intervalto detect when the P-waves and T-waves are approaching each other ineither amplitude or time. A baseline amplitude difference and/or abaseline T-P interval may be established at block 444 for use by controlmodule 206 in detecting a decrease in either the amplitude difference orthe T-P interval.

If the second waveform is not within the T-wave sensing window (“no”branch of block 414) and the first and second waveforms do match (“yes”branch of block 428), the waveforms could be two P-waves, two T-waves ora P-wave and a T-wave that are indistinct from each other. In thissituation, an analysis of the unfiltered signal 265 or an alternatefiltered signal 267 is performed at block 434 to determine if T-wavescan be identified from the unfiltered signal 255 or alternate filteredsignal 257 and whether they coincide in time with the first or nextP-wave sensing threshold crossings detected at blocks 402 and 410,respectively. In other examples, the alternate signal analyzed at block434 to identify a T-wave coinciding in time with the first or nextP-wave sensing threshold crossings is an integrated signal, such assignal 480 in FIG. 7B, in which the T-wave 486 is enhanced compared tothe T-wave 466 in the filtered EGM signal 460. A T-wave identified fromthe unfiltered signal 255, alternate filtered signal 257, or anintegrated signal (of the filtered signal 266 or unfiltered signal 265)may be determined to coincide in time with the first or next P-wavesensing threshold crossings when the identified T-wave occurs within apredetermined interval of time from the P-wave sensing thresholdcrossing.

In some cases, a T-wave may not be readily identified from theunfiltered EGM signal 265. Filter 264 may be adjusted to increase theT-wave amplitude in an alternative filtered EGM signal 267. Multiplefilter adjustments may be performed until a T-wave can be identified.For example up to six different center frequency and bandwidthcombinations may be tested to determine if the T-wave can be identifiedfrom the alternate filtered EGM signal 267. Sensing module 202 maydetermine if T-waves can be identified within a maximum number of filteradjustments in some examples.

If a T-wave is identified from the unfiltered signal 265 or alternatefiltered EGM signal 267 but is not identified from the filtered EGMsignal 266 at approximately the same time, the P-wave detector filter264 is optimally filtering T-waves from the filtered EGM signal 266.P-wave sensing is confirmed at block 436 in response to identifyingT-waves from the unfiltered EGM signal 265 (or alternate filtered EGMsignal 267) that are absent from the filtered EGM signal 266. Both ofthe P-wave sensing threshold crossings of the filtered EGM signal 266are P-waves that match each other morphologically and both are properlydetected outside the T-wave sensing window. No adjustment of the T-wavesensing window or the P-wave detector filter 264 is needed. Confirmationis made that P-waves are being properly sensed and discriminated fromT-waves at block 432.

If T-waves can be identified and correspond in time to one or both ofthe first and next P-wave sensing threshold crossings of the filteredEGM signal 266, adjustment of the T-wave sensing window and/or theP-wave detector filter 264 is required to improve P-wave and T-wavediscrimination based on at least one of timing, amplitude or morphology.If T-waves can be identified, and correspond in time to one or bothP-wave sensing threshold crossings, T-waves are not being optimallyfiltered from the EGM signal and are confounding P-wave sensing. Sensingmodule 204 adjusts at least one of the T-wave sensing window and/or theP-wave detector filter 264 at block 438 to improve separation anddiscrimination of P-waves and T-waves.

In some cases, T-waves and P-waves may not be distinguished from eachother based on comparisons made between the first and second waveformsof the filtered EGM signal 266 at block 418 or based on analysis of theunfiltered signal 265 or alternate EGM signal 267 at block 434 such thatP-wave sensing cannot be confirmed at block 436. Adjustment of theT-wave sensing window may be performed at block 438 to improveseparation of the signals, but in some cases T-wave sensing windowadjustment may not improve discrimination between T-waves and P-waves.The T-wave and P-wave may be overlapping, particularly if the heart rateis elevated from a resting heart rate. The P-wave detector filter 264may be adjusted at block 438 to improve the P-wave signal strength,diminish the T-wave signal strength or intentionally increase the T-waveamplitude to be greater than the P-wave amplitude in the filtered EGMsignal 266. Adjustment to the P-wave detector filter 264 and/or T-wavesensing window are performed at block 438 so that P- and T-waves can bediscriminated at block 432.

At block 438 (and block 426), adjustment to the P-wave detector filter264 for increasing amplitude, waveform or timing separation of theP-wave and T-wave in the filtered EGM signal 266 may include adjustmentof the center frequency and/or the bandpass width. For example, thefilter may be adjusted to a bandpass centered on the T-wave frequency tointentionally increase T-wave amplitude but with a wide enough bandpassthat includes P-waves, or centered on the P-wave frequency to increaseP-wave amplitude with a narrow bandwidth that eliminates T-waves. Thebandpass may be symmetrical or asymmetrical to intentionally increasesignal strength of P-waves relative to T-waves or vice versa.

The center frequency may be selected in the range of 10 Hz to 50 Hz witha bandwidth including frequencies in the range of 5 Hz up to 70 Hz insome examples. The bandwidth may be relatively narrow or wide, e.g., atotal bandwidth of up to 50 Hz or as narrow as 10 Hz. For example,filter 264 may initially be adjusted to a center frequency of 20 Hz witha symmetrical 30 Hz bandpass width for a total 3 dB range of 5 Hz to 35Hz to maximize the P-wave amplitude. If the T-wave is indistinct fromthe P-wave using this filtering, the filter center frequency may beadjusted lower or higher, the bandpass width may be increased ordecreased, and/or the bandpass may be shifted to an asymmetricalbandpass that has a greater range less than the center frequency or agreater range higher than the center frequency. For example, the filter264 may be shifted to a center frequency of 25 Hz with a narrow bandpasswidth of 20 Hz for a total 3 dB range of 15 Hz to 35 Hz to remove orattenuate a lower frequency T-wave from the filtered EGM signal 266. Insome examples, the P-wave may be a narrow signal having a higher thannormal frequency, so that filter 264 may be adjusted to a 40 Hz centerfrequency in this situation with a relatively narrow bandwidth. In someinstances, the center frequency of P-wave detector filter 264 isselected to include both P-waves and R-waves within the filter bandpass.In other instances, the P-wave detector center frequency may be centeredon an expected P-wave frequency.

Once the P-wave detector filter 264 is adjusted to achieve P- and T-wavediscrimination at block 432 based on amplitude, time, or othermorphology features in the filtered EGM signal 266, which may includerepeated adjustments of filter 264 until an optimal center frequency andbandpass is identified for separating P-wave and T-wave signals, P-wavesensing may be enabled at block 442. The P-wave sensing criteria may beset at block 444 based on the discrimination achieved by filteradjustments. After adjusting the P-wave detector filter 264, the P-wavesignal may be altered. As such, the P-wave signal features used todefine the P-wave sensing criteria may be re-determined, compared toanalogous T-wave features and used to establish P-wave sensing criteriaat block 444.

In some cases, reliable discrimination of P-waves and T-waves will notbe achieved at block 432, despite multiple filter adjustments and/or oneor more T-wave sensing window adjustments. If this occurs, the RVpacemaker 14 may be set to a temporary VVI pacing mode at block 440 bycontrol module 206 in which P-wave sensing is disabled for use insetting AV pacing escape intervals. The control module 206 controlsventricular pacing pulse delivery by the pulse generator 202 in a singlechamber ventricular pacing and sensing mode (i.e., a VVI mode) where VVescape intervals are set in response to R-sense signals 258 andventricular pacing pulses. An R-sense signal 258 during a VV escapeinterval inhibits a scheduled pacing pulse and restarts the VV escapeinterval. P-wave detector 262 is either disabled by control module 206or P-sense signals 272 produced by P-wave detector 262 are ignored bypace timing and control module 270 for setting ventricular pacing escapeintervals.

The VVI pacing mode set at block 440 may be temporary. Periodically, thesensing module 204 may return to block 438 to adjust the T-wave windowand/or P-wave detector filter 264 to achieve reliable discrimination ofP-waves and T-waves based on amplitude, timing and/or morphology. Forexample, the sensing module 204 may return to block 438 every 10seconds, every 30 seconds, every one minute, or other predetermined timeinterval to attempt adjustments to separate the P-waves and T-waves. Ifreliable discrimination is achieved (block 432), the control module 206switches from the temporary VVI pacing mode back to an atrialsynchronized ventricular pacing mode in which P-wave sensing is enabledat block 442, and the P-wave sensing criteria are updated by sensingmodule 204 as needed at block 444. The pace timing and control module270 of control module 206 sets ventricular pacing escape intervals to AVpacing escape intervals in response to P-sense signals 272 forcontrolling ventricular pacing pulse delivery by pulse generator 202.

FIG. 9 is a flow chart 500 of a method for controlling P-wave sensingand ventricular pacing by RV pacemaker 14 according to another example.The techniques shown in flow chart 500 may be performed as needed forestablishing P-wave sensing criteria by sensing module 202. Thetechniques of flow chart 500 may be performed for confirming P-wave andT-wave discrimination, e.g., at block 432 of FIG. 8, or duringadjustments to the T-wave sensing window or the P-wave detector filter264 for improving discrimination between P-waves and T-waves. Inparticular, the techniques of flow chart 500 may be performed when theT-wave and P-wave are determined to be indistinct during R-wave sensingby sensing module 204. The method of flow chart 500 and other flowcharts presented herein for setting P-wave sensing criteria andcontrolling ventricular pacing using sensed P-waves may be performedcooperatively by sensing module 204 and control module 206.

At block 502, the P-wave detector 262 detects a P-wave sensing thresholdcrossing. A test AV escape interval is started at block 504 in responseto the P-wave sensing threshold crossing as long as the EGM signal doesnot reach the R-wave sensing threshold (or an intermediate T-wavesensing threshold if used) during the time limit (310 in FIG. 5A). TheEGM signal waveform associated with the P-wave sensing thresholdcrossing is stored over an analysis window at block 506. The test AVescape interval set at block 504 may be a shorter interval than atargeted AV interval that is used by the pace timing and control module270 during bradycardia or rate responsive pacing. The shorter intervalis used in order to promote ventricular pacing pulse delivery prior toan intrinsic R-wave during the process of flow chart 500. For example,the AV interval may be shortened by 100 ms. Shortening of the AVinterval also promotes temporal separation of the P-wave and the T-wavefor facilitating discrimination and analysis of the two separatewaveforms.

During the test AV interval, the EGM signal is monitored for an R-wavesensing threshold crossing by R-wave detector 252 at block 508. If theP-wave sensing threshold crossing detected at block 502 is followed byan R-sense signal at block 508 during the test AV interval, the test AVescape interval is cancelled at block 510.

If a V-sense signal does not occur during the AV interval, the test AVinterval expires, and a ventricular pacing pulse is delivered at block512. After delivering the ventricular pacing pulse, the next P-wavesensing threshold crossing is detected at block 514 (outside anyrelevant ventricular blanking period after the ventricular pacing pulsewhich may be set to include the pacing-evoked R-wave). In the methodshown by flow chart 500, a waveform crossing the P-wave sensingthreshold stored prior to the ventricular pacing pulse is expected to bea P-wave and the first waveform crossing the P-wave sensing thresholdafter the ventricular blanking period is expected to be a T-wave. Assuch, a comparative analysis of a pre-pace waveform and a post-pacewaveform enables P-wave sensing criteria to be established thatdiscriminate P-waves from T-waves.

If a T-wave sensing threshold crossing occurs within the time limit fromthe second P-wave sensing threshold crossing, as determined at block515, the T-wave of the pacing evoked response is distinct from theP-wave based on T-wave amplitude. Atrial-synchronous ventricular pacingis enabled at block 528. No further adjustment may be needed to reliablydiscriminate between P-waves and T-waves.

If a T-wave sensing threshold crossing does not occur within the timelimit, the P-wave and pacing-evoked T-wave may be indistinct. Furtheranalysis is required to determine if reliable P-wave and T-wavediscrimination can be achieved. The EGM signal is stored during ananalysis window at block 516. The analysis windows set at blocks 506 and516 for storing the EGM signal waveform after a P-wave sensing thresholdcrossing may be equal in duration and may begin and end at the sametimes relative to the respective P-wave sensing threshold crossings,which may include buffered sample points stored prior to the P-wavesensing threshold crossing.

The stored waveforms are compared at block 518 by performing amorphology comparison of the overall waveform of all the signal samplepoints during the analysis window and/or by determining and comparingspecific analogous features of the waveforms during the analysis window,such as peak amplitude, signal width, slope etc. If the waveforms do notmatch, as determined at block 520, the P-waves and T-waves aredetermined to be distinguishable.

A T-wave sensing window is adjusted as necessary at block 524 based onthe timing of the P-wave sensing threshold crossing at block 514 and/ordetermining the timing of the T-wave sensing threshold crossing duringthe analysis window of the post-pace (second) stored waveform. TheT-wave sensing window may be lengthened, shortened, or shifted in timerelative to the ventricular pacing pulse to promote a high likelihood ofthe second P-wave sensing threshold crossing occurring during the T-wavesensing window following pacing pulses and intrinsic R-waves. It isrecognized that the Q-T interval following a pacing pulse may bedifferent than the Q-T interval during an intrinsic ventricular beat. Assuch, the T-wave sensing window may be set to account for thisdifference.

At block 526, P-wave sensing criteria are set by sensing module 204based on the comparison performed at block 518. One or more features maybe identified that have the greatest difference between the pre-pacewaveform (P-wave) and the post-pace waveform (T-wave). The greatestdifference may be identified as a greatest percentage difference or agreatest normalized difference. The P-wave sensing criteria may be setto include a threshold or range that includes the P-wave value for afeature having a greatest difference and excludes the analogous T-wavefeature value. The difference between the overall morphologies of thewaveform as defined by all of the equally spaced sample points acrossthe analysis windows may be used to set a threshold percentagedifference between the P-wave and T-wave morphology for discriminatingbetween P-waves and T-waves.

The T-wave sensing window and the P-wave sensing criteria set at blocks524 and 526, respectively, may be stored in memory 210 and used byP-wave detector 262 for confirming P-waves during AV escape intervalsset in response to detecting P-wave sensing threshold crossings. TheP-wave sensing threshold may be included in the P-wave sensing criteriaset at block 526. The P-wave sensing threshold may be set as apercentage of the peak amplitude of the waveform stored at block 506.The T-wave sensing threshold may additionally or alternatively be set atblock 526 by sensing module 204 based on the amplitude of the post-pacestored waveform and/or a difference between the peak amplitude of thefirst, pre-pace stored waveform (block 404) and the second, post-pacestored waveform (block 412).

If the two waveforms match at block 520, the T-wave arising from theevoked response to the ventricular pacing pulse may be indistinguishablefrom the P-wave sensed prior to the ventricular pacing pulse. In thiscase, P-wave sensing may not be reliable for use in controllingventricular pacing. The P-wave detector filter 264 may be adjusted atblock 522 until the pre-pace P-wave and the post-pace T-wave no longerhave matching morphologies as determined at block 530. The process ofadjusting the filter may include multiple iterations of adjusting thecenter frequency and/or bandpass, collecting waveforms of the P-wave andthe post-pace T-wave, and comparing the waveforms. In some cases, ananalysis of the P-wave frequency or signal width and/or an analysis ofthe T-wave frequency or signal width may be performed to guide thefilter adjustment to reduce the number of times adjustments are madeuntil successful P-wave and T-wave discrimination is made. In otherexamples, a set of predefined combinations of center frequency andbandpass width may be defined and tested at block 522 until acombination corresponding to a maximum difference in amplitude, signalwidth or other feature of the P-wave and T-wave is identified.

If successful P-wave and T-wave discrimination is achieved by adjustingthe filter properties, based on non-matching waveforms as determined atblock 530, the T-wave sensing window may be adjusted at block 524 toinclude the T-wave. P-wave sensing criteria are set at block 526 basedon the features determined from the P-wave and T-wave of the EGM signalafter filter adjustments. Atrial-synchronous ventricular pacing isenabled at block 528.

In some cases, filter adjustments may not successfully yield a distinctP-wave and T-wave. The P-wave, however, may cause an inflection point inthe descending portion of the T-wave when the P-wave is overlapping theT-wave. An inflection point in the descending portion of the T-wave maybe identified as the P-wave in some examples. A P-wave inflection pointmay be identified based on previously stored T-wave morphology at lowerheart rates. The matching waveform comprising indistinct T- and P-wavesmay be compared to the previously stored T-wave morphology. In oneexample, the number of inflection points of the T-wave occurring after amaximum peak T-wave of the filtered cardiac signal is stored. During anyof the processes disclosed herein, when P-waves and T-waves aredetermined to be indistinct in a filtered or unfiltered cardiac signalbased on time, amplitude or separate P-wave and T-wave morphologies,identifying a P-wave inflection point along a descending portion of theT-wave can be performed for discriminating the P-wave from the T-waveand establishing P-wave sensing criteria.

If a new inflection point can be identified in the descending portion ofthe T-wave at block 531 that was not present in the T-wave at a lowerheart rate, this new inflection point may be evidence of the P-wave andused for setting P-wave sensing criteria at block 526 during high heartrates associated with overlapping P- and T-waves. Atrial-synchronizedventricular pacing may be enabled at block 528 based on the ability tosense a P-wave inflection in the descending portion of the T-wave.

If the filter adjustments do not successfully yield a distinct P-waveand T-wave and a P-wave inflection along the descending portion of theT-wave cannot be identified, the control module 206 sets the pacing modeof the RV pacemaker 14 to a temporary single chamber ventricular pacingmode (VVI) that does not set AV pacing escape intervals using P-wavesensing at block 532. Alternatively, the control module 206 may set atemporary refractory period that encompasses an overlapping T-wave andP-wave so that the overlapping events occurring within the refractoryperiod are both ignored for the purposes of setting an AV pacing escapeinterval.

A relatively short test AV interval set at block 504 promotes temporalseparation of the P-wave and the T-wave since the R-wave and subsequentT-wave will both occur earlier when the test AV interval is shortened.In some examples, the test AV interval may be shortened at block 504 ifthe P-wave and T-wave are indistinct based on amplitude, time, and/ormorphology. Before setting a temporary VVI (or VVIR) mode at block 531,all or a portion of the flow chart of FIG. 9 may be performed with ashortened AV test interval to determine if the P-wave and T-wave aredistinct.

An interval between a ventricular pacing pulse and the P-wave sensingthreshold crossing can be measured when different test AV escapeintervals are applied. As described below in conjunction with FIG. 12,modulating the timing of a ventricular pacing pulse, e.g., by modulatingthe test AV escape interval, causes the interval between the ventricularpacing pulse and the next P-wave sensing threshold crossing to changewhen the P-wave sensing threshold crossing is a true P-wave. Thisinterval will not change, however, when the P-wave sensing thresholdcrossing is a T-wave during a stable heart rate. As such, in someexamples, all or a portion of FIG. 9 may be repeated using differenttest AV pacing escape intervals to separate the P-wave and T-wave foruse in establishing P-wave sensing criteria and enablingatrial-synchronous ventricular pacing. If the P-wave sensing thresholdcrossing after the ventricular pacing pulse is verified as a P-wavebased on an altered interval from the ventricular pacing pulse to theP-wave sensing threshold crossing during different test AV escapeintervals, atrial-synchronous ventricular pacing may be enabled at block528.

If the interval to the next P-wave threshold crossing after aventricular pacing pulse is not altered by modulation of the test AVescape interval, the P-wave threshold crossing detected at block 514after the ventricular pacing pulse is a T-wave. If amplitude ormorphology-based P-wave sensing criteria can be established at block 526to distinguish the confirmed T-wave from P-waves, atrial-synchronousventricular pacing can be enabled at block 528. If P-wave sensingcriteria cannot be established to distinguish the confirmed T-wave fromthe P-wave when the AV escape interval is restored to a target AV pacingescape interval, the temporary VVI(R) pacing mode is set at bock 532.

It is recognized that while the process shown in FIG. 9 indicates asingle waveform stored at block 506 prior to a ventricular pacing pulseand a single post-pace waveform stored at block 516, the process ofcollecting pre- and post-pace waveforms may be repeated for multiplepacing cycles to obtain average pre- and post-pace waveforms that arecompared at block 518 and/or to obtain multiple differences fromcomparisons made between multiple pairs of pre- and post-pace waveformsthat can be used to determine an average difference.

To illustrate, a pre-pace waveform may be stored at block 506 and apost-pace waveform may be stored at block 516 for a predetermined numberof consecutive or non-consecutive ventricular pacing pulses, e.g., threeto eight ventricular pacing pulses, delivered at the test AV escapeinterval. The pre- and post-pace waveforms may be compared for eachpacing cycle to determine waveform feature differences. The threewaveform features having the greatest differences for all of the pairedpre- and post-pace waveforms may be selected as features fordiscriminating between P-waves and T-waves. The minimum differencedetermined from all of the comparisons for a selected feature may beused as a basis for setting the P-wave sensing criteria. Examples ofwaveform features that may be determined include, but are not limitedto, signal width, peak amplitude, peak slope, frequency content, andwavelet or Haar transform coefficients.

After establishing the P-wave sensing criteria, the criteria may beverified by testing the criteria against additionally acquired pre- andpost-pace waveforms to verify that each of the pre- and post-pacewaveforms are correctly confirmed as P-waves and T-waves respectively.After establishing and setting the P-wave sensing criteria at block 526,atrial synchronous ventricular pacing is enabled at block 528. P-wavesensing threshold crossings are used to start AV pacing escape intervalsand P-waves confirmed during the AV pacing escape intervals using theP-wave sensing criteria allow the AV pacing escape intervals to time outfor triggering ventricular pacing pulse delivery.

Additionally, baseline P-wave and T-wave amplitude measurements andbaseline T-P interval measurements may be determined and stored at block526 after setting the P-sense criteria. As described in conjunction withFIG. 10A, the control module 206 may be configured to use a baselineamplitude difference and/or the baseline T-P interval to detect adecrease in the amplitude difference between P-waves and T-waves or adecrease in the T-P interval.

FIG. 10A is a flow chart 550 of a method for determining a need foradjusting the P-wave detector filter 264 and/or T-wave sensing window.In some examples, control module 206 is configured to determine when theP-wave and T-wave are likely to become indistinguishable before theybecome indistinguishable. The P-wave and T-wave amplitudes or relativetime from each other may change with heart rate, electrode position,patient position, or other factors. Control module 206 may be configuredto determine when the T-wave and P-wave are approaching each other intime before they become overlapping and indistinguishable based on time.Control module 206 may additionally or alternatively be configured todetermine when the amplitudes of the P-wave and T-wave are approachingeach other before they become indistinguishable based on amplitude. Inthis way, the control module 206 may be enabled to control P-wavedetector 262 to adjust filter 264 and/or the T-wave sensing window inadvance of the P-wave and the T-wave becoming indistinguishable tomaintain reliable P-wave sensing.

At block 552, control module 206 is configured to monitor T-P intervals,i.e., the time interval between the T-wave and the far-field P-wave. Forexample, a T-P interval may be determined between a cancel P-sensesignal 274 produced by P-wave detector 262 (when the R-wave sensingthreshold is not crossed within the time limit) and the next P-sensesignal 272 (shown in FIG. 4A). As described in conjunction with FIG. 5C,a cancel P-sense signal 274 may be produced in response to a T-wavesensing threshold crossing within time limit 310 of a P-wave sensingthreshold crossing. The next EGM signal after a T-wave is expected to bea P-wave. As such, a time interval from the cancel P-sense signal 274 tothe next P-sense signal 272 produced in response to the next P-wavesensing threshold crossing may be determined by control module 206 as aT-P interval. In other examples, a T-P interval may be determined fromthe unfiltered or alternate filtered EGM signal 265 or 267 when theT-wave is substantially filtered from the filtered EGM signal 266.

The T-P interval may be monitored beat-by-beat or on a less frequentbasis to detect a shortening of the T-P interval. If the T-P interval isshortening, overlapping T-waves and P-waves may become indistinct fromeach other. A decrease in the T-P interval is detected at block 554.Detection of a shortened or decreased T-P interval may be based oncomparing the T-P interval to a threshold interval or detecting apercentage change in the T-P interval compared to a previously measuredor baseline T-P interval. In various examples, a single beat or runningaverage T-P interval may be determined and if the single beat or runningaverage T-P interval is decreasing in value for three or more successivedeterminations, a shortening T-P interval is detected at block 554. Ifthe T-P interval shortens to a threshold interval or other criteria fordetecting T-P interval shortening are met, the P-wave sensing criteriamay be checked at block 560.

At block 556, control module 206 is configured to monitor an amplitudedifference between sensed far-field P-waves and near-field T-waves. Forexample, a T-wave maximum peak amplitude may be determined during asignal analysis window after a cancel P-sense signal 274 is produced inresponse to the filtered EGM signal 266 crossing the T-wave sensingthreshold as shown and described in conjunction with FIG. 5C. In otherexamples, the T-wave may be filtered from the filtered EGM signal 266such that the T-wave is smaller than the P-wave. In this case, theT-wave maximum peak amplitude may be determined during the T-wavesensing window.

The P-wave maximum peak amplitude may be determined during a signalanalysis window applied after the next P-sense signal 272 produced inresponse to the next crossing of the P-wave sensing threshold whenanother higher threshold crossing does not occur within time limit 310(as shown in FIG. 5A). The difference between the T-wave amplitude andthe P-wave amplitude may then be determined on a beat-by-beat or lessfrequent basis or as a running average amplitude difference.

A decreased amplitude difference is detected at block 558. The decreasedamplitude difference may be due to a change in one or both of the P-waveamplitude and the T-wave amplitude. The decrease may be detected whenthe amplitude difference is less than a threshold difference or hasdecreased a predetermined percentage from a previously measureddifference or baseline difference. In some examples a running averageamplitude difference is compared to a baseline difference. The decreasemay alternatively be detected at block 558 in response to a requirednumber of consecutively determined differences being less than apreceding difference. In response to detecting a decreased amplitudedifference, the P-wave sensing criteria are checked at block 560.

Checking P-wave sensing criteria may include performing the process offlow chart 400 (FIG. 8), performing the process of flow chart 500 (FIG.9), or any portion or combination thereof. In some examples, the P-wavesensing criteria are checked at block 560 by adjusting the P-wavedetector filter to increase the amplitude of one of the P-wave orT-wave, decrease the amplitude of one of the P-wave or T-wave, or causea change in the filtered EGM signal morphology that results in anincrease in the T-P interval. If filter adjustments do not provideincreased P-wave and T-wave separation based on amplitude or time, theT-wave sensing window may be adjusted at block 560 to improvediscrimination between the T-wave and the P-wave based on time. T-wavemorphology of the filtered EGM signal 266 may be compared with orwithout filter adjustments to set new P-wave sensing criteria thatprovide reliable morphology discrimination for verifying P-waves duringthe AV interval.

FIG. 10B is a flow chart 570 of a method for determining a need foradjusting the P-wave sensing criteria according to another example. Atblock 571, if ventricular pacing is currently being delivered at thetarget AV pacing escape interval, the V-pace to P-sense interval isdetermined. The V-pace to P-sense interval is the time interval from adelivered ventricular pacing pulse to the next P-sense signal that isconfirmed to be a P-wave during the AV pacing escape interval based onany additionally applied P-wave sensing criteria other than a P-wavesensing threshold crossing (as described above in conjunction with FIGS.7A and 7B. If ventricular pacing is not currently being delivered whenthe process of flowchart 570 is started due to R-sense event signalsoccurring during the AV interval, a shortened test AV interval may beapplied at block 571 to establish an initial V-pace to P-sense interval.

At block 572, the AV pacing escape interval is modulated from the targetAV pacing escape interval. The AV pacing escape interval may be shortedin one step change, e.g., shortened by up to 100 ms, or modulated in twoor more step changes to AV intervals that are shorter than the target AVinterval to maintain ventricular pacing pulse delivery. The V-pace toP-sense interval is re-determined at block 574 for one or more alteredAV intervals applied at block 572.

At block 576, the control module 206 determines if the V-pace to P-senseinterval is altered in response to the modulated AV interval. If theV-pace to P-sense intervals match for two different AV pacing escapeintervals, T-wave oversensing is suspected as determined at block 582.Control module 206 initiates a process to check and re-establish theP-wave sensing criteria at block 584, e.g., using the techniquesdescribed above in conjunction with FIG. 9. If the V-pace to P-senseinterval does change with modulation of the AV interval, as determinedat block 576, P-wave sensing is confirmed at block 578.Atrial-synchronized ventricular pacing continues at block 580 using thetarget AV interval and currently established P-wave sensing criteria.

FIG. 11A is a diagram 600 of a cardiac EGM signal 601 and associatedmarker channel signals 620 that may be produced by RV pacemaker 14 andtransmitted to the external device 20 shown in FIG. 1 according to oneexample. FIG. 11B is a diagram 650 of the cardiac EGM signal 601transmitted from the RV pacemaker 14 to the external device 20 and anassociated marker channel display 640 that may be produced by a userdisplay of external device 20.

In FIGS. 11A and 11B, EGM signal 601 includes P-waves 602, ventricularpacing pulses 604 and T-waves 605. A P-wave sensing threshold 615 isindicated but may or may not be displayed graphically in a displayproduced by external device 20. In some examples, a numerical value ofthe P-wave sensing threshold may be displayed.

Since R-waves, T-waves 605, noise, and other signals may cross theP-wave sensing threshold 615, the RV pacemaker 14 confirms that a P-wavesensing threshold crossing 603 is a true P-wave 602 by comparing the EGMsignal 601 to P-wave sensing criteria. As described above, the EGMsignal 601 may be compared to a T-wave sensing threshold and/or anR-wave sensing threshold during a time limit as described in conjunctionwith FIGS. 5A-5C for use in controlling (maintaining, cancelling oradjusting) an AV pacing escape interval 612 that is started upon theP-wave sensing threshold crossing 603. Additionally or alternatively,the EGM signal 601 may be stored during an analysis window 606 andcompared to P-wave sensing criteria to verify that the P-wave sensingthreshold crossing is due to a true P-wave 602.

Verification that the EGM signal 601 does not cross a T-wave sensingthreshold and/or an R-wave sensing threshold during the time limit 310(shown in FIGS. 5A-5C) and/or storing of the EGM signal waveform duringan analysis window 606 and comparing the stored waveform to P-wavesensing criteria will require time after the P-wave sensing thresholdcrossing 603. Actual confirmation of the P-wave sensing thresholdcrossing as a true P-wave sensed event is, therefore, delayed in time bya delay time interval 610 after the P-wave sensing threshold crossing603. The delay time interval 610 required to confirm a P-wave sensedevent after the P-wave sensing threshold crossing 603 may be on theorder of 20 ms to 120 ms, for example, but is within the AV pacingescape interval 612. The delay time interval 610 may equal the durationof an analysis window 606 or be longer than the analysis window 606.

After P-wave detector 262 verifies that the P-wave sensing thresholdcrossing 603 is due to a true P-wave 602, the RV pacemaker controlmodule 206 may produce a marker channel P-wave sense event signal 608.The marker channel P-wave sense event (AS) signal 608 is produced afterthe time delay 610, upon confirming the P-wave sense event.

If the P-wave 602 is confirmed by additional signal analysis after theP-wave sensing threshold crossing 603, the AV pacing escape interval 612started upon the P-wave sensing threshold crossing 603 is allowed toexpire. A ventricular pacing pulse 604 is delivered upon expiration ofthe AV pacing escape interval 612. The RV pacemaker 14 may produce amarker channel ventricular pacing pulse (VP) signal 614 aligned in timewith the ventricular pacing pulse 604.

If the external device 20 produces a display of EGM signal 601 and theAS and VP marker channel signals 608 and 614 in real time, the AS signal608 appears to occur at the delay time interval 610 after the actualP-wave 602. The ventricular pacing pulse signal 614 appears to occur ata very short AV interval 618 after the AS signal 608 instead of at theactual AV pacing escape interval 612. The relative timing of markerchannel signals 608 and 614 to each other and the EGM signal 601 asproduced in real time may create user confusion if the EGM signal 601and marker channel signals 612 and 614 appear in real time in a userdisplay generated by external device 20.

In order to avoid user confusion, the control module 206 delaystransmission of the EGM signal 601 by the pacemaker telemetry module 208by a delay time 630 that is greater than or equal to the delay timeinterval 610 between P-wave sensing threshold crossing 603 andgeneration of the marker channel P-wave sense event signal 608. As shownin FIG. 10B, the user display of external device 20 produces a displayof EGM signal 601 that is delayed from real time by delay time interval630.

The marker channel 640 produced by the external device 20 may displaythe AS event marker 628 in real time when the marker channel P-wavesense event signal 608 is produced and transmitted from RV pacemaker 14.The VP marker 628, however, is displayed at the delay time 630 from theVP signal 614 produced by RV pacemaker 14. In this way, the P-wavesensing threshold crossing 603 and the start of the AV pacing escapeinterval 612 appear to be aligned with the P-wave sense event marker 628even though the AS signal 608 is generated by RV pacemaker 14 later thanthe actual P-wave sensing threshold crossing 603. By delaying the VPmarker 634 in real time from the pacing pulse signal 614 by delay time630, the pacing pulse marker 634 is displayed at the expected AV pacingescape interval 612 after the AS event marker 628 and remains alignedwith the pacing pulse 604 appearing on EGM signal 601, which has alsobeen delayed by the same delay time 630. In this way, the markers 628and 634 are properly aligned with corresponding events, i.e., P-wave 602and pacing pulse 604, of the EGM signal 601 to enable straight-forwardand logical analysis of the sensing and pacing operations of RVpacemaker 14.

FIG. 12 is a flow chart 700 of a method performed by RV pacemaker 14 forautomatically adjusting the P-wave detector filter 264 according toanother example. At block 702, the RV pacemaker control module 206determines if it is time for selecting or adjusting the bandpass ofP-wave detector filter 264. Bandpass selection may be performedaccording to the method of flow chart 700 on a periodic basis when theheart rhythm is verified to be a normal sinus rhythm, e.g., based onR-wave sense event signals 258 (FIG. 4A). Bandpass selection mayadditionally or alternatively be performed according to the method offlow chart 700 on a triggered basis. For example, the process shown byflow chart 700 may be entered from block 560 of FIG. 10A or block 584 ofFIG. 10B.

At block 704, the control module 206 may control pulse generator 202 ina temporary safe pacing mode, e.g., a VVI pacing mode set at a nominalpacing rate or a rate based on a median or running average of RRintervals between R-wave sense event signals 258. During the process forsearching for an optimal P-wave detector filter bandpass, ventricularpacing may be controlled by control module 206 such that pacing pulsedelivery is unaffected by over or under sensing of cardiac events thatmay occur as the P-wave detector filter 264 is adjusted. In other words,pace timing module 270 may be temporarily configured to respond only toR-wave sense event signals 258 from R-wave detector 252 for controllingthe timing of ventricular pacing pulses during the process of flow chart700.

At block 706, the sensing module 204 of RV pacemaker 14 determines ifT-waves are distinct from P-waves and R-waves in the filtered cardiacelectrical signal 266 at the current filter bandwidth. T-waves aredetermined to be distinct from both P-waves and R-waves based onpredetermined distinction criteria. For example, T-wave distinctioncriteria may require that the differences between a T-wave feature andthe analogous P-wave and R-wave features are sufficiently large enoughto confidently set sensing thresholds between these feature values thatreliably distinguish P-waves, T-waves and R-waves in the filtered signal266.

In some cases, T-waves may be identifiable from the filtered cardiacelectrical signal 266 but may not meet criteria defined for determiningthat the T-waves are distinct. For example, T-waves may be identifiedfrom the filtered signal 266 based on timing following an R-wave senseevent signal from R-wave detector 252, e.g., during a T-wave window.However, in the filtered signal 266, the difference between the T-wavepeak amplitude and the P-wave peak amplitude may not meet T-wavedistinction criteria. For example, T-wave and P-wave peak amplitudes maybe required to have at least a 50% difference to determine that theidentified T-wave is distinct from the P-wave. In another example, theT-wave and the P-wave may be identified following the R-wave but may bemerged such that T-wave and the P-wave are not separated enough in timeor amplitude to meet predetermined criteria for detecting the T-waves asbeing distinct.

The determination of whether the T-wave is distinct may be made at block706, for example, based on a comparison of the maximum peak amplitudesof a P-wave, R-wave and T-wave each sensed using respective sensingthresholds according to the methods described in conjunction with FIGS.5A through 5C. If the maximum peak amplitudes of each sensed event are apredetermined difference away from each other, e.g., at least 50% to 75%away from each other enabling distinct cardiac event sensing thresholdamplitudes to be defined, the P-, T- and R-waves may be determined to bedistinct at block 706. The relative time of a sensed P-wave and a sensedT-wave from a sensed R-wave may also be used to determine distinctnessbetween the three cardiac events at block 706. In some cases, a T-waveis determined to be distinct from P-waves and R-waves at block 706 basedon an absence of a T-wave within a T-wave window following the R-waveand a P-wave occurring outside the T-wave window. In this case, theT-wave may be effectively filtered from the filtered signal 266.

If the T-waves are distinct from P-waves and R-waves at block 706, thecurrent bandpass of the P-wave detector filter 264 is determined to beappropriate at block 732. No adjustment of the filter bandpass is made.The sensing module 204 and control module 206 may exit the process offlow chart 700, and switch from the safe pacing mode set at block 704 toreturn to atrial-synchronized ventricular pacing using FF sensedP-waves. The pace timing and control module 270 may be re-enabled torespond to P-wave sense event signals 272.

If the T-waves are not distinct from P-waves and R-waves at block 706according to predetermined criteria for classifying the T-waves as beingdistinct, the sensing module 204 determines if T-waves can be identifiedfrom the filtered cardiac signal 266 at block 708. If T-waves are notidentified at block 708, e.g., if only R-waves and P-waves are detectedat block 706, the sensing module 204 or the control module 206 may checkfor evidence of possible T-wave oversensing (TWOS) at block 710.

For example, if a fast ventricular rate is detected at block 710, basedon a rate of R-wave sense event signals 258 produced by R-wave detector252 or based on a rate of R-wave sensing threshold crossings determinedby P-wave detector 262, T-wave oversensing may be occurring. In somecases, T-waves may be sensed as R-waves and correctly identified asT-waves due to the T-wave amplitude being indistinct from the R-waveamplitude. A fast rate of sensed R-waves may be evidence of TWOS. A fastrate of sensed R-waves may be detected based on RR intervals less than400 ms, less than 300 ms, or less than a predefined tachycardiadetection interval.

In other instances, T-waves may be oversensed as P-waves due to thetiming and amplitude of T-waves and P-waves being indistinct based oncurrently established P-wave sensing criteria. In this case, the rhythmmay be AS-VP-AS-VP occurring at a relatively fast rate, for example at aVP-AS interval of 400 ms. The AS events may be T-waves falsely sensed asP-waves and causing the pace timing and control module 270 to set AVpacing escape intervals in response to the false P-wave sense eventsignals. The T-waves falsely sensed as P-waves can lead to pacing theventricle at a relatively fast rate. Accordingly the rhythm pattern ofAS-VP-AS-VP at regular short intervals may be evidence of TWOS detectedat block 710.

In order to determine if TWOS is occurring in response to determiningthat TWOS is suspected at block 710, the P-wave detector 262 temporarilyincreases the P-wave sensitivity and/or modulates the ventricular pacingpulse timing at block 712. P-wave sensitivity may be increased bylowering the P-wave sensing threshold. The ventricular pacing pulsetiming may be modulated by adjusting the VV escape interval and/oradjusting the AV escape interval. If the AV escape interval isshortened, the next P-wave sense event is expected to be later after theR-wave, but a true T-wave after the R-wave is expected to move earlierwith the earlier ventricular pacing pulse. If an atrial sense eventoccurs earlier upon shortening the AV escape interval, TWOS is detected.The earlier atrial sense event is identified as an oversensed T-wave atblock 714. Bandpass filter adjustment is made at block 720 or 728 asdescribed below to make the T-wave and P-wave distinct.

In another example, the AV escape interval may be lengthened, e.g., by50 ms or more. The time interval between the ventricular pacing pulseand the atrial sense event is determined at the lengthened AV escapeinterval. If the time interval from the ventricular pacing pulse to thenext atrial sense event stays the same, the atrial sense event is anoversensed T-wave. Bandpass filter adjustment is made at block 720 or728 as described below to make the T-wave and P-wave distinct. If theinterval shortens, the atrial sense event is a true atrial event; theventricular pacing pulse has been moved later or closer to the nextatrial event in time.

T-waves are identified at block 714 if modulation of the ventricularpacing pulse timing does not alter a V-pace to P-Sense interval (inwhich case the P-Sense is actually a T-wave). T-waves may also beidentified if a lower P-wave sensing threshold results in more atrialsense events. True P-waves may be sensed at the increased sensitivityand true T-waves may be identified as being oversensed as P-waves whenthe sensitivity is lower (sensing threshold higher). If oversensedT-waves are identified, bandpass filter adjustment is made by advancingto block 722.

In other instances, T-waves may be sensed at a lower P-wave sensingthreshold when the T-wave has a lower amplitude than the P-waves. Iflowering the P-wave sensing threshold reveals low amplitude sensedevents consistent with T-wave timing relative to R-wave sense events inaddition to true P-wave sense events, T-waves are identified at block714, and T-wave filtering is deemed appropriate at block 732.

If T-waves can still not be identified at block 714 at the modulatedventricular pacing pulse times and/or reduced P-wave sensing threshold,T-waves may be considered absent from the filtered signal 266. TheP-wave detector filter bandpass is deemed appropriate at block 732. Noadjustment of the bandpass is required, and atrial-synchronizedventricular pacing may be restored. Additionally or alternatively,T-waves may be identified at block 714 using a differential signaland/or an integrated signal as described above in conjunction with FIG.7B.

If T-wave oversensing is not suspected based on the ventricular rate atblock 710, or if T-waves are identified but are similar to P-waves inamplitude, timing and/or morphology as determined at block 716, thesensing module 204 determines at block 718 if the P-wave detector filter264 has already been adjusted to maximize the T-wave. The process 700performed by RV pacemaker 14 takes advantage of the expected bandpassfrequencies that will increase the T-wave signal in the filtered cardiacelectrical signal 266 in order to intentionally increase T-waveamplitude when the T-wave amplitude and/or morphology is similar to theP-wave.

If the filter has not been adjusted to maximize the T-wave signal, thebandpass of filter 264 is adjusted at block 720 to increase the T-wavesignal strength. The bandpass may be adjusted, e.g., by lowering thecenter frequency and/or increasing or decreasing the bandwidth. If thenew bandpass makes P-waves, T-waves and R-waves distinct, as determinedat block 730, e.g., based on the criteria described above in conjunctionwith block 706, the adjusted bandpass is deemed appropriate at block732. The T-wave may be intentionally increased in amplitude by adjustingthe bandpass of P-wave detector filter 264 to provide greater separationof the P-wave and T-wave based on amplitude, time, and or morphology.

While not explicitly shown in FIG. 12, it is understood that the processmay repeat the steps at block 718 and 720 to make multiple adjustmentsto the P-wave detector filter 264 until the T-wave signal is increasedenough to make T-waves distinct from P-waves. If the filter has beenadjusted to a bandpass that results in a maximum T-wave signal amplitudeand/or width at block 718, but the T-waves are still not distinct fromP-waves, the bandpass may be adjusted at block 728 to make the T-wavesignals smaller in the filtered signal 266.

Similarly, if the T-waves are identified in the filtered signal at block708, and are distinct from P-waves but are similar to the R-waves inamplitude as determined at block 724, the bandpass may be adjusted atblock 728 to make the T-wave signal strength smaller. While notexplicitly shown in FIG. 12, the steps at blocks 728 and 722 foradjusting the bandpass to decrease the T-wave signal strength in thefiltered signal 266 until the T-wave cannot be made any smaller may berepeated multiple times until a bandpass that results in the minimizedT-wave signal is identified.

At block 730, the sensing module 204 determines if the adjusted bandpassmakes the R-wave, P-wave and T-wave distinct in the filtered signal bydecreasing the T-wave signal, not necessarily to a minimum. If theP-wave, T-wave, and R-wave are now distinct, e.g., by making the T-waveamplitude smaller, the adjusted bandpass is deemed appropriate forenabling P-wave sensing and atrial-synchronized ventricular pacing atblock 732. Sensing module 204 may be configured to determine if T-wavesare distinct from P-waves and from R-waves within a maximum number offilter adjustments.

If T-waves can be identified from the filtered signal at block 708 butT-wave features are distinct from P-waves but not R-waves based onpredetermined distinction criteria (“no” branch of block 724), thebandpass may be adjusted to increase the R-wave amplitude at block 726to increase amplitude-based separation between T-waves and R-waves insome examples. In some cases, separation of R-waves and T-waves byP-wave detector 262 is not required if the separate R-wave detector 252is reliably producing R-wave sense event signals 258. However, if allthree events are being sensed from filtered signal 266 and used forcontrolling pacing escape interval, it may be desirable to increase theseparation between T-waves and R-waves when these signals areindistinct. If separation between T-waves and R-waves is increased,T-waves and P-waves can be distinguished based on time and/or amplituderelative to a sensed R-wave.

If all adjusted bandpasses tested for either increasing or decreasingthe T-wave signal strength or increasing the R-wave fail to make theP-wave, T-wave, and R-wave distinct based on at least time, amplitudeand/or morphology, as determined at block 730, according topredetermined distinction criteria, the control module 206 maytemporarily suspend atrial-synchronized ventricular pacing at block 734by setting the pacing mode to a VVI mode. The bandpass of filter 264 maybe reverted back to a previous setting until a future attempt atadjusting the bandpass filter is made again.

While the disclosure is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the disclosure to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the disclosure as defined by the appended claims.

For example, the following Items are illustrative of furtherembodiments:

Item 1. A method performed by a medical device, comprising:

filtering a raw cardiac electrical signal received by the medical deviceaccording to first filtering properties to produce a filtered cardiacelectrical signal, the raw cardiac electrical signal comprising firstcardiac events, second cardiac events different than the first cardiacevents, and third cardiac events different than the first cardiac eventsand the second cardiac events;

detecting a first crossing of a first threshold by the filtered cardiacelectrical signal;

identifying one of the second cardiac events after the first crossing;

detecting a second crossing of the first threshold by the filteredcardiac electrical signal after the identified one of the second cardiacevents;

analyzing the first crossing and the second crossing of the filteredcardiac electrical signal;

establishing cardiac event sensing criteria that discriminate the firstcardiac events from the third cardiac events based on the analyzing ofthe first crossing and the second crossing; and

sensing the first cardiac events from the filtered cardiac electricalsignal when the established cardiac event sensing criteria are met.

Item 2. The method of item 1, further comprising identifying the one ofthe second cardiac events in response to one of a delivered cardiacpacing pulse and a crossing of a second threshold by a cardiacelectrical signal, the second threshold greater than the firstthreshold.Item 3. The method of any one of items 1-2, wherein analyzing the firstcrossing and the second crossing comprises:

setting a sensing window in response to identifying the second cardiacevent; and

determining if one of the first crossing and the second crossing occurswithin the sensing window.

Item 4. The method of any one of items 1-3, wherein analyzing the firstcrossing and the second crossing of the filtered cardiac electricalsignal comprises determining an alternate signal, the alternate signalbeing at least one of a differential signal of the filtered cardiacelectrical signal, a differential signal of the raw cardiac electricalsignal, an integrated signal of the filtered cardiac electrical signal,and an integrated signal of the raw cardiac electrical signal; and

comparing a feature of the alternate signal at a time of the firstcrossing to a feature of the alternate signal at a time of the secondcrossing.

Item 5. The method of any one of items 1-4, wherein:

analyzing the first crossing and the second crossing of the filteredcardiac electrical signal comprises:

-   -   acquiring a first waveform of the filtered cardiac electrical        signal corresponding to the first crossing of the first        threshold,    -   acquiring a second waveform of the filtered cardiac electrical        signal corresponding to the second crossing of the first        threshold, and    -   comparing the first waveform to the second waveform to verify        that the first and second waveforms do not match; and

establishing the cardiac event sensing criteria comprises:

-   -   determining a first feature from the first waveform;    -   determining a second feature from the second waveform, the        second feature analogous to the first feature;    -   determining a difference between the first feature and the        second feature; and    -   establishing the cardiac event sensing criteria by at least        setting a cardiac event sensing criterion based on the        difference.        Item 6. The method of any one of items 1-5, wherein:        analyzing the first crossing and the second crossing comprises:

setting a sensing window in response to identifying the second cardiacevent;

determining if the second crossing occurs within the sensing window; and

determining if the first waveform and the second waveform match; andestablishing the cardiac event sensing criteria to include setting thesensing window in response to identified second cardiac events when thesecond crossing occurs within the sensing window and the first waveformand the second waveform do not match.

Item 7. The method of any one of items 1-5, wherein analyzing the firstcrossing and the second crossing comprises:

determining if the first waveform and the second waveform match;

analyzing an alternate cardiac electrical signal when the first waveformand the second waveform do match, the alternate cardiac electricalsignal being one of the raw cardiac electrical signal, an integratedsignal of the filtered cardiac electrical signal, and an alternatefiltered cardiac electrical signal produced by filtering the raw cardiacelectrical signal according to second filtering properties differentthan the first filtering properties;

identifying the third cardiac event on the alternate cardiac electricalsignal;

determining if one of first crossing and the second crossing coincidewith the third cardiac event on the alternate cardiac electrical signal;and

adjusting the first filtering properties when one of the first thresholdcrossing and the next threshold crossing coincide with the third cardiacevent of the alternate cardiac electrical signal.

Item 8. The method of any one of items 1-7, wherein analyzing the firstcrossing and the second crossing further comprises:

determining if the first waveform and the second waveform match based onthe comparing;

searching for an inflection point along a descending portion of at leastone of the first waveform and the second waveform when the firstwaveform and the second waveform match;

detecting one of the first cardiac events along one of the third cardiacevents in the filtered cardiac electrical signal when the inflectionpoint is found along the descending portion; and

establishing the cardiac event sensing criteria in response to findingthe inflection point along the descending portion.

Item 9. The method of any one of items 1-8, further comprising:

when the first waveform and the second waveform do not match based onthe comparing, enabling setting a pacing escape interval upon sensingthe first cardiac events according to the established sensing criteria;

controlling a pulse generator to deliver a pacing pulse to a patient'sheart when the pacing escape interval expires;

and

disabling setting the pacing escape interval when the first waveform andthe second waveform do match based on the comparing.

Item 10. The method of any one of items 1-9, further comprising:

detecting a decreasing time interval between the third cardiac eventsand the first cardiac events, and

updating the established sensing criteria in response to detecting thedecreasing event time interval.

Item 11. The method of any one of items 1-10, further comprising:

setting a first pacing escape interval in response to sensing a firstsensed event sensed according to the established sensing criteria;

delivering a first pacing pulse upon expiration of the first pacingescape interval;

determining a first time interval from the first pacing pulse to asecond sensed event sensed according to the established sensingcriteria;

setting a second pacing escape interval in response to sensing a thirdsensed event sensed according to the established sensing criteria;

delivering a second pacing pulse upon expiration of the second pacingescape interval, the second pacing escape interval shorter than thefirst pacing escape interval;

determining a second time interval from the second pacing pulse to afourth sensed event sensed according to the established sensingcriteria;

determining if the first time interval matches the second time interval;and

updating the established sensing criteria in response to the first timeinterval and the second time interval matching.

Item 12. An implantable medical device, comprising:

a sensing module configured to receive a raw cardiac electrical signalvia electrodes coupled to the sensing module, the raw cardiac electricalsignal comprising first cardiac events, second cardiac events differentthan the first cardiac events, and third cardiac events different thanthe first cardiac events and the second cardiac events, the sensingmodule configured to:

filter the raw cardiac electrical signal according to first filteringproperties to produce a filtered cardiac electrical signal;

detect a first crossing of a first threshold by the filtered cardiacelectrical signal;

identify one of the second cardiac events after the first crossing;

detect a second crossing of the first threshold by the filtered cardiacelectrical signal after the identified one of the second cardiac events;

analyze the first crossing and the second crossing of the filteredcardiac electrical signal;

establish cardiac event sensing criteria that discriminate the firstcardiac events from the third cardiac events based on the analyzing ofthe first crossing and the second crossing; and

sense the first cardiac events from the filtered cardiac electricalsignal when the established cardiac event sensing criteria are met.

Item 13. The device of item 12, further comprising a pulse generatorconfigured to generate and deliver a pacing pulse to a patient's heartvia electrodes coupled to the pulse generator,

wherein the sensing module is configured to identify the one of thesecond cardiac events in response to one of a delivered cardiac pacingpulse and a crossing of a second threshold by a cardiac electricalsignal, the second threshold greater than the first threshold.

Item 14. The device of any one of items 12-13, wherein the sensingmodule is configured to analyze the first crossing and the secondcrossing by at least:

setting a sensing window in response to identifying the second cardiacevent;

and

determining if one of the first crossing and the second crossing occurswithin the sensing window.

Item 15. The device of any one of items 12-14, wherein the sensingmodule is configured to analyze the first crossing and the secondcrossing of the filtered cardiac electrical signal by:

determining an alternate signal, the alternate signal being at least oneof a differential signal of the filtered cardiac electrical signal, adifferential signal of the raw cardiac electrical signal, an integratedsignal of the filtered cardiac electrical signal, and an integratedsignal of the raw cardiac electrical signal; and

comparing a feature of the alternate signal at a time of the firstcrossing to a feature of the alternate signal at a time of the secondcrossing.

Item 16. The device of any one of items 12-15, wherein the sensingmodule is configured to analyze the first crossing and the secondcrossing of the filtered cardiac electrical signal by at least:

acquiring a first waveform of the filtered cardiac electrical signalcorresponding to the first crossing of the first threshold;

acquiring a second waveform of the filtered cardiac electrical signalcorresponding to the second crossing of the first threshold; and

comparing the first waveform to the second waveform.

Item 17. The device of any one of items 12-16, wherein the sensingmodule is further configured to:

analyze the first crossing and the second crossing by:

-   -   setting a sensing window in response to identifying the second        cardiac event;    -   determining if the second crossing occurs within the sensing        window; and    -   determining if the first waveform and the second waveform match;        and

establishing the cardiac event sensing criteria to include setting thesensing window in response to identified second cardiac events when thesecond crossing occurs within the sensing window and the first waveformand the second waveform do not match.

Item 18. The device of any one of items 12-17, wherein the sensingmodule is further configured to analyze the first crossing and thesecond crossing by:

determining if the first waveform and the second waveform match;

analyzing an alternate cardiac electrical signal received by the sensingmodule via the electrodes coupled to the sensing module when the firstwaveform and the second waveform do match, the alternate cardiacelectrical signal being one of the raw cardiac electrical signal, anintegrated signal of the filtered cardiac electrical signal, and analternate filtered cardiac electrical signal produced by the sensingmodule by filtering the raw cardiac electrical signal according tosecond filtering properties different than the first filteringproperties;

identifying the third cardiac event on the alternate cardiac electricalsignal;

determining if one of first crossing and the second crossing coincidewith the third cardiac event on the alternate cardiac electrical signal;and

adjusting the first filtering properties when one of the first thresholdcrossing and the next threshold crossing coincide with the third cardiacevent of the alternate cardiac electrical signal.

Item 19. The device of any one of items 12-18, wherein the sensingmodule is further configured to analyze the first crossing and thesecond crossing by:

determining if the first waveform and the second waveform match based onthe comparing;

searching for an inflection point along a descending portion of at leastone of the first waveform and the second waveform when the firstwaveform and the second waveform match;

detecting one of the first cardiac events along one of the third cardiacevents in the filtered cardiac electrical signal when the inflectionpoint is found along the descending portion; and

establishing the cardiac event sensing criteria in response to findingthe inflection point along the descending portion.

Item 20. The device of any one of items 12-19, further comprising:

a pulse generator configured to generate and deliver a pacing pulse to apatient's heart via electrodes coupled to the pulse generator;

a control module coupled to the sensing module and the pulse generatorand configured to:

-   -   enable setting a pacing escape interval when the first waveform        and the second waveform do not match based on the comparing;    -   set the pacing escape interval upon sensing each one of the        first cardiac events according to the established sensing        criteria;    -   control the pulse generator to deliver a pacing pulse to a        patient's heart when the pacing escape interval expires; and    -   disable setting the pacing escape interval when the first        waveform and the second waveform do match based on the        comparing.        Item 21. The device of any one of items 12-20, wherein the        sensing module is further configured to:

detect a decreasing event time interval between the third cardiac eventsand the first cardiac events, and

update the established sensing criteria in response to detecting thedecreasing event time interval.

Item 22. The device of any one of items 12-21, further comprising:

a pulse generator configured to generate and deliver a pacing pulse to apatient's heart via electrodes coupled to the pulse generator;

a control module coupled to the sensing module and the pulse generatorand configured to:

set a first pacing escape interval in response to a first sensed eventsensed by the sensing module according to the established sensingcriteria;

control the pulse generator to deliver a first pacing pulse uponexpiration of the first pacing escape interval;

determine a first time interval from the first pacing pulse to a secondsensed event sensed by the sensing module according to the establishedsensing criteria;

set a second pacing escape interval in response to a third sensed eventsensed by the sensing module according to the established sensingcriteria;

delivering a second pacing pulse upon expiration of the second pacingescape interval, the second pacing escape interval shorter than thefirst pacing escape interval;

determining a second time interval from the second pacing pulse to afourth sensed event sensed by the sensing module according to theestablished sensing criteria;

determining if the first time interval matches the second time interval;and

updating the established sensing criteria in response to the first timeinterval and the second time interval matching.

Item 23. A non-transitory, computer-readable medium storing a set ofinstructions which, when executed by an implantable medical device causethe device to:

filter a raw cardiac electrical signal received by the medical deviceaccording to first filtering properties to produce a filtered cardiacelectrical signal, the raw cardiac electrical signal comprising firstcardiac events, second cardiac events different than the first cardiacevents, and third cardiac events different than the first and secondcardiac events;

detect a first crossing of a first threshold by the filtered cardiacelectrical signal;

identify one of the second cardiac events after the first crossing;

detect a second crossing of the first threshold by the filtered cardiacelectrical signal after the identified one of the second cardiac events;

analyze the first crossing and the second crossing of the filteredcardiac electrical signal;

establish cardiac event sensing criteria that discriminate the firstcardiac events from the third cardiac events based on the analyzing ofthe first crossing and the second crossing; and

sense the first cardiac events from the filtered cardiac electricalsignal when the established cardiac event sensing criteria are met.

Thus, various examples of an implantable medical device including asensing extension having a flotation member have been described. It isrecognized that various modifications may be made to the describedembodiments without departing from the scope of the following claims.

1. A method performed by a medical device, comprising: filtering a rawcardiac electrical signal received by the medical device according tofirst filtering properties to produce a filtered cardiac electricalsignal, the raw cardiac electrical signal comprising first cardiacevents, second cardiac events different than the first cardiac events,and third cardiac events different than the first cardiac events and thesecond cardiac events; detecting a first crossing of a first thresholdby the filtered cardiac electrical signal; identifying one of the secondcardiac events after the first crossing; detecting a second crossing ofthe first threshold by the filtered cardiac electrical signal after theidentified one of the second cardiac events; analyzing the firstcrossing and the second crossing of the filtered cardiac electricalsignal; establishing cardiac event sensing criteria that discriminatethe first cardiac events from the third cardiac events based on theanalyzing of the first crossing and the second crossing; and sensing thefirst cardiac events from the filtered cardiac electrical signal whenthe established cardiac event sensing criteria are met.
 2. The method ofclaim 1, further comprising identifying the one of the second cardiacevents in response to one of a delivered cardiac pacing pulse and acrossing of a second threshold by a cardiac electrical signal, thesecond threshold greater than the first threshold.
 3. The method ofclaim 1, wherein analyzing the first crossing and the second crossingcomprises: setting a sensing window in response to identifying thesecond cardiac event; and determining if one of the first crossing andthe second crossing occurs within the sensing window.
 4. The method ofclaim 1, wherein analyzing the first crossing and the second crossing ofthe filtered cardiac electrical signal comprises determining analternate signal, the alternate signal being at least one of adifferential signal of the filtered cardiac electrical signal, adifferential signal of the raw cardiac electrical signal, an integratedsignal of the filtered cardiac electrical signal, and an integratedsignal of the raw cardiac electrical signal; and comparing a feature ofthe alternate signal at a time of the first crossing to a feature of thealternate signal at a time of the second crossing.
 5. The method ofclaim 1, wherein: analyzing the first crossing and the second crossingof the filtered cardiac electrical signal comprises: acquiring a firstwaveform of the filtered cardiac electrical signal corresponding to thefirst crossing of the first threshold, acquiring a second waveform ofthe filtered cardiac electrical signal corresponding to the secondcrossing of the first threshold, and comparing the first waveform to thesecond waveform to verify that the first and second waveforms do notmatch; and establishing the cardiac event sensing criteria comprises:determining a first feature from the first waveform; determining asecond feature from the second waveform, the second feature analogous tothe first feature; determining a difference between the first featureand the second feature; and establishing the cardiac event sensingcriteria by at least setting a cardiac event sensing criterion based onthe difference.
 6. The method of claim 5, wherein: analyzing the firstcrossing and the second crossing comprises: setting a sensing window inresponse to identifying the second cardiac event; determining if thesecond crossing occurs within the sensing window; and determining if thefirst waveform and the second waveform match; and establishing thecardiac event sensing criteria to include setting the sensing window inresponse to identified second cardiac events when the second crossingoccurs within the sensing window and the first waveform and the secondwaveform do not match.
 7. The method of claim 5, wherein analyzing thefirst crossing and the second crossing comprises: determining if thefirst waveform and the second waveform match; analyzing an alternatecardiac electrical signal when the first waveform and the secondwaveform do match, the alternate cardiac electrical signal being one ofthe raw cardiac electrical signal, an integrated signal of the filteredcardiac electrical signal, and an alternate filtered cardiac electricalsignal produced by filtering the raw cardiac electrical signal accordingto second filtering properties different than the first filteringproperties; identifying the third cardiac event on the alternate cardiacelectrical signal; determining if one of first crossing and the secondcrossing coincide with the third cardiac event on the alternate cardiacelectrical signal; and adjusting the first filtering properties when oneof the first threshold crossing and the next threshold crossing coincidewith the third cardiac event of the alternate cardiac electrical signal.8. The method of claim 5, wherein analyzing the first crossing and thesecond crossing further comprises: determining if the first waveform andthe second waveform match based on the comparing; searching for aninflection point along a descending portion of at least one of the firstwaveform and the second waveform when the first waveform and the secondwaveform match; detecting one of the first cardiac events along one ofthe third cardiac events in the filtered cardiac electrical signal whenthe inflection point is found along the descending portion; andestablishing the cardiac event sensing criteria in response to findingthe inflection point along the descending portion.
 9. The method ofclaim 5, further comprising: when the first waveform and the secondwaveform do not match based on the comparing, enabling setting a pacingescape interval upon sensing the first cardiac events according to theestablished sensing criteria; controlling a pulse generator to deliver apacing pulse to a patient's heart when the pacing escape intervalexpires; and disabling setting the pacing escape interval when the firstwaveform and the second waveform do match based on the comparing. 10.The method of claim 1, further comprising: detecting a decreasing timeinterval between the third cardiac events and the first cardiac events,and updating the established sensing criteria in response to detectingthe decreasing event time interval.
 11. The method of claim 1, furthercomprising: setting a first pacing escape interval in response tosensing a first sensed event sensed according to the established sensingcriteria; delivering a first pacing pulse upon expiration of the firstpacing escape interval; determining a first time interval from the firstpacing pulse to a second sensed event sensed according to theestablished sensing criteria; setting a second pacing escape interval inresponse to sensing a third sensed event sensed according to theestablished sensing criteria; delivering a second pacing pulse uponexpiration of the second pacing escape interval, the second pacingescape interval shorter than the first pacing escape interval;determining a second time interval from the second pacing pulse to afourth sensed event sensed according to the established sensingcriteria; determining if the first time interval matches the second timeinterval; and updating the established sensing criteria in response tothe first time interval and the second time interval matching.
 12. Animplantable medical device, comprising: a sensing module configured toreceive a raw cardiac electrical signal via electrodes coupled to thesensing module, the raw cardiac electrical signal comprising firstcardiac events, second cardiac events different than the first cardiacevents, and third cardiac events different than the first cardiac eventsand the second cardiac events, the sensing module configured to: filterthe raw cardiac electrical signal according to first filteringproperties to produce a filtered cardiac electrical signal; detect afirst crossing of a first threshold by the filtered cardiac electricalsignal; identify one of the second cardiac events after the firstcrossing; detect a second crossing of the first threshold by thefiltered cardiac electrical signal after the identified one of thesecond cardiac events; analyze the first crossing and the secondcrossing of the filtered cardiac electrical signal; establish cardiacevent sensing criteria that discriminate the first cardiac events fromthe third cardiac events based on the analyzing of the first crossingand the second crossing; and sense the first cardiac events from thefiltered cardiac electrical signal when the established cardiac eventsensing criteria are met.
 13. The device of claim 12, further comprisinga pulse generator configured to generate and deliver a pacing pulse to apatient's heart via electrodes coupled to the pulse generator, whereinthe sensing module is configured to identify the one of the secondcardiac events in response to one of a delivered cardiac pacing pulseand a crossing of a second threshold by a cardiac electrical signal, thesecond threshold greater than the first threshold.
 14. The device ofclaim 12, wherein the sensing module is configured to analyze the firstcrossing and the second crossing by at least: setting a sensing windowin response to identifying the second cardiac event; and determining ifone of the first crossing and the second crossing occurs within thesensing window.
 15. The device of claim 12, wherein the sensing moduleis configured to analyze the first crossing and the second crossing ofthe filtered cardiac electrical signal by: determining an alternatesignal, the alternate signal being at least one of a differential signalof the filtered cardiac electrical signal, a differential signal of theraw cardiac electrical signal, an integrated signal of the filteredcardiac electrical signal, and an integrated signal of the raw cardiacelectrical signal; and comparing a feature of the alternate signal at atime of the first crossing to a feature of the alternate signal at atime of the second crossing.
 16. The device of claim 12, wherein thesensing module is configured to analyze the first crossing and thesecond crossing of the filtered cardiac electrical signal by at least:acquiring a first waveform of the filtered cardiac electrical signalcorresponding to the first crossing of the first threshold; acquiring asecond waveform of the filtered cardiac electrical signal correspondingto the second crossing of the first threshold; and comparing the firstwaveform to the second waveform.
 17. The device of claim 16, wherein thesensing module is further configured to: analyze the first crossing andthe second crossing by: setting a sensing window in response toidentifying the second cardiac event; determining if the second crossingoccurs within the sensing window; and determining if the first waveformand the second waveform match; and establishing the cardiac eventsensing criteria to include setting the sensing window in response toidentified second cardiac events when the second crossing occurs withinthe sensing window and the first waveform and the second waveform do notmatch.
 18. The device of claim 16, wherein the sensing module is furtherconfigured to analyze the first crossing and the second crossing by:determining if the first waveform and the second waveform match;analyzing an alternate cardiac electrical signal received by the sensingmodule via the electrodes coupled to the sensing module when the firstwaveform and the second waveform do match, the alternate cardiacelectrical signal being one of the raw cardiac electrical signal, anintegrated signal of the filtered cardiac electrical signal, and analternate filtered cardiac electrical signal produced by the sensingmodule by filtering the raw cardiac electrical signal according tosecond filtering properties different than the first filteringproperties; identifying the third cardiac event on the alternate cardiacelectrical signal; determining if one of first crossing and the secondcrossing coincide with the third cardiac event on the alternate cardiacelectrical signal; and adjusting the first filtering properties when oneof the first threshold crossing and the next threshold crossing coincidewith the third cardiac event of the alternate cardiac electrical signal.19. The device of claim 16, wherein the sensing module is furtherconfigured to analyze the first crossing and the second crossing by:determining if the first waveform and the second waveform match based onthe comparing; searching for an inflection point along a descendingportion of at least one of the first waveform and the second waveformwhen the first waveform and the second waveform match; detecting one ofthe first cardiac events along one of the third cardiac events in thefiltered cardiac electrical signal when the inflection point is foundalong the descending portion; and establishing the cardiac event sensingcriteria in response to finding the inflection point along thedescending portion.
 20. The device of claim 16, further comprising: apulse generator configured to generate and deliver a pacing pulse to apatient's heart via electrodes coupled to the pulse generator; a controlmodule coupled to the sensing module and the pulse generator andconfigured to: enable setting a pacing escape interval when the firstwaveform and the second waveform do not match based on the comparing;set the pacing escape interval upon sensing each one of the firstcardiac events according to the established sensing criteria; controlthe pulse generator to deliver a pacing pulse to a patient's heart whenthe pacing escape interval expires; and disable setting the pacingescape interval when the first waveform and the second waveform do matchbased on the comparing.
 21. The device of claim 12, wherein the sensingmodule is further configured to: detect a decreasing event time intervalbetween the third cardiac events and the first cardiac events, andupdate the established sensing criteria in response to detecting thedecreasing event time interval.
 22. The device of claim 12, furthercomprising: a pulse generator configured to generate and deliver apacing pulse to a patient's heart via electrodes coupled to the pulsegenerator; a control module coupled to the sensing module and the pulsegenerator and configured to: set a first pacing escape interval inresponse to a first sensed event sensed by the sensing module accordingto the established sensing criteria; control the pulse generator todeliver a first pacing pulse upon expiration of the first pacing escapeinterval; determine a first time interval from the first pacing pulse toa second sensed event sensed by the sensing module according to theestablished sensing criteria; set a second pacing escape interval inresponse to a third sensed event sensed by the sensing module accordingto the established sensing criteria; delivering a second pacing pulseupon expiration of the second pacing escape interval, the second pacingescape interval shorter than the first pacing escape interval;determining a second time interval from the second pacing pulse to afourth sensed event sensed by the sensing module according to theestablished sensing criteria; determining if the first time intervalmatches the second time interval; and updating the established sensingcriteria in response to the first time interval and the second timeinterval matching.
 23. A non-transitory, computer-readable mediumstoring a set of instructions which, when executed by an implantablemedical device cause the device to: filter a raw cardiac electricalsignal received by the medical device according to first filteringproperties to produce a filtered cardiac electrical signal, the rawcardiac electrical signal comprising first cardiac events, secondcardiac events different than the first cardiac events, and thirdcardiac events different than the first and second cardiac events;detect a first crossing of a first threshold by the filtered cardiacelectrical signal; identify one of the second cardiac events after thefirst crossing; detect a second crossing of the first threshold by thefiltered cardiac electrical signal after the identified one of thesecond cardiac events; analyze the first crossing and the secondcrossing of the filtered cardiac electrical signal; establish cardiacevent sensing criteria that discriminate the first cardiac events fromthe third cardiac events based on the analyzing of the first crossingand the second crossing; and sense the first cardiac events from thefiltered cardiac electrical signal when the established cardiac eventsensing criteria are met.